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00:00Beneath the surface of our planet lies a vast realm.
00:06Look at this.
00:07Countless caves, hidden tunnels, and submerged rivers.
00:15Now, on a quest for knowledge, expert cavers are descending into the uncharted abyss.
00:22Descending!
00:24Where extraordinary discoveries lie waiting.
00:27Looks like a very kind of violent act.
00:30Armed with the latest scanning technology to penetrate the darkness.
00:35Here it is.
00:36Look at all the detail. This is amazing.
00:40And turn data into accurate 3D images.
00:46This time, in the Mediterranean, can scanning a huge sea cave and a labyrinth of tiny passages
00:55unravel a 2,000-year-old mystery?
00:59Whatever's through there could be even more important than the discoveries we've found here so far.
01:04Can diving a perilous underwater chamber reveal how it formed deep inside a mountain?
01:11Cave divers contend with a number of risks. You either survive or you don't.
01:15And will a man-made cave complex surrender its secrets?
01:20The whole story just makes you wanna, you know, get your caving helmet on, plow through those tunnels.
01:24And expose the inspiration for the world's most famous spy.
01:28The Underworld team is heading to a unique location.
01:47Lying at the southern tip of the Iberian Peninsula is a tiny territory of just under 3 square miles.
02:02Gibraltar. A British overseas territory famous for its rock.
02:09A huge limestone monolith which towers 1,000 feet over one of the busiest shipping lanes in the world.
02:20Its position at the mouth of the Mediterranean has long made Gibraltar important.
02:26Not only as a strategic base for the modern military, but throughout history people have left their mark on this enigmatic place that sits between oceans and continents.
02:40It may look solid, but Gibraltar's rock has one of the highest concentrations of caves and man-made tunnels anywhere on Earth.
02:48Including a UNESCO World Heritage Site that has yielded extraordinary artifacts connected to early types of man.
03:01To help unravel these mysteries, local scientists have called in Ru Walters, one of the world's leading cave surveyors.
03:09He uses cutting edge technology to create detailed scans of caves, mines, and even sewers.
03:22The scientists hope his data would explain why ancient people believed something terrifying lurked inside this rock.
03:31It really is one huge lump of limestone.
03:34On a radio I can see little pot marks all over the cliffs and things.
03:39This is going to be a very, very exciting trip.
03:55Stuart!
03:56Ru.
03:57Good to see you.
03:58Nice to finally meet you.
03:59Ru has been invited here by Stuart Finlayson, who grew up exploring Gibraltar's caves.
04:06Both of his parents are prominent scientists here, and Stuart has followed in their footsteps.
04:13I think you're going to be surprised at just how many caves we have here.
04:18Stuart is taking Ru to a large cave complex, lying on the south-eastern coast.
04:24A highly protected site, it has limited public access.
04:37So welcome to Gorham's Cave, Ru.
04:39It's probably the most important cave we have in Gibraltar.
04:43Archaeology spanning about 120,000 years.
04:46It's all so beautiful.
04:48Yeah.
04:49It really is.
04:50It's all so beautiful.
04:53Discovered in 1907, this cave's archaeological importance has long been known.
04:59But it's only within the last few decades that systematic excavation has been carried out.
05:06Artifacts from many periods of history have been found.
05:10Ranging from the tools of early human species to the treasures of ancient civilizations.
05:16We know, Ru, that caves have attracted people over time.
05:21That's happened all over the world.
05:22But there's obviously something pretty special going on in this cave.
05:28Most caves you get one, two meters of archaeology.
05:31Yeah.
05:32A few shards here and there.
05:34But Gorham's, we have 18 meters of deposit left.
05:38So it's like a snapshot in time.
05:39Stuart hopes a scan will reveal new details in parts of the cave out of reach from his team.
05:48I think we're going to have to scan this in a much finer resolution than we normally do.
05:53Yeah.
05:54So you can get some real forensic data.
05:55That's exactly what we need.
05:56That's exactly what we need.
05:57To have to shed some new light on what's going on.
05:59Looking forward to it.
06:00Let's see.
06:04Although the team have excavated parts of the main chamber, a tight passage at the back of the cave has always eluded them.
06:13Stuart thinks it could hold clues to understanding the cave's importance to ancient peoples.
06:19But needs the scan to confirm his theories and justify what would be a difficult and delicate excavation.
06:26So, Ru, we're at the deepest part of Gorham's cave.
06:31As you can see from that tiny little hole, it's a passageway going even further.
06:37It gets pretty tight, as you can see.
06:39We haven't been able to work in there yet, but we do know that in this entire area all around us, there's a lot of symbolism from different kinds of peoples over time.
06:49Whatever's through there could actually be even more important than the discoveries that we've found here so far.
06:54You're certainly quite a challenge there, aren't you?
06:58It's quite a snug hole.
07:00It is, it is a bit, yeah.
07:01I'm not the thin, thin, slim caver I used to be.
07:05It's a challenge and half that to get through there, I think.
07:11Despite its extraordinary contents, Gorham's cave has never before been mapped in such detail.
07:18Ru and his colleague John will be using a cutting edge 3D scanner to create a highly detailed digital model of the entire cave.
07:30It will send out 1.2 million pulses of light each second to accurately capture every detail within inches.
07:38Certainly looking forward to getting up there and having a look at that part, because that's the business end of the cave where all the history is.
07:51Recent analysis of excavated finds from the back of Gorham's cave has revealed one of the strangest objects ever unearthed here.
07:59So this is one of the puzzles from the back of the cave near that dark entrance.
08:13What we have found deep within the deposits are essentially these two eyes.
08:26They're ceramic.
08:28We have multiple fragments.
08:30And when you put them all together, they create a mask.
08:34What we've been able to do is actually reconstruct what that mask would have looked like.
08:40It's a mask representing the Gorgon Medusa.
08:46Medusa was an ancient Greek monster.
08:50One of three hideous sisters with snakes for hair.
08:54Legend has it that one look could turn you straight to stone.
08:59It almost certainly would have been hanging on the wall.
09:02It was clearly made by the Greeks as it is a Greek goddess.
09:05There were ancient mariners passing through the straits all the time, but we know that they were in the area from anchors and so on that have been found within the sea.
09:16But why would ancient Greek mariners have left this mask of Medusa in Gorham's cave?
09:23Could they have believed this was Medusa's lair?
09:26It's almost as if people were trying to say, go no further.
09:32But this mask is not the only one of its kind.
09:36Many other stone faces of Medusa made by the ancient Greeks have been found all over the eastern Mediterranean.
09:43Usually left as votive offerings in temples or worn as amulets to ask the gods for protection.
09:52The Gorham's cave mask is different.
09:56It is the only one found on the western Mediterranean and the only one found within a cave.
10:02Stuart hopes Rue's scan will reveal the significance of Gorham's cave to the ancient Greeks once and for all.
10:10So what we want to understand is, what was so special about this cave?
10:15What was it about this location?
10:17Why did they leave this mask here?
10:19It's possible that the key to unlocking this mystery is at the back of that cave.
10:30The ancient Greeks may have left their mark on Gibraltar's caves, but they weren't the first to use them.
10:37Stuart's father, Clive Finlayson, is the director of the Gibraltar National Museum.
10:47His research looks at some of this area's earliest inhabitants.
10:51So we know that Gibraltar's been inhabited for at least 120,000 years, and this particular very, very special specimen that we have in the museum is the key to knowing that.
11:06This skull is a replica of the original.
11:10Now in the Natural History Museum in London, it was found in a quarry here in 1848.
11:16There were a number of features that made this look odd in comparison to humans today.
11:23The eyes are very big for a start.
11:25Instead of a flat forehead, you have these very pronounced eyebrows, if you like.
11:31And when you turn it round, you can see that the skull goes backwards, tapers back, whereas in ours it has a much more pronounced forehead.
11:38But it wasn't until eight years later, when a second, similar skull was discovered over 1,500 miles away in the Neander Valley, that its true significance was eventually recognized.
11:51Scientists had found a whole new species, one of our closest extinct relatives, Homo neanderthalensis, or the Neanderthal.
12:03While our ancestors were evolving in Africa, Neanderthals were already living in Eurasia.
12:14But after the Homo sapiens started moving to Europe around 40,000 years ago, the Neanderthals began to die out.
12:22The popular belief, as early depictions show, is they were ape-like creatures, lacking the intelligence to compete with early humans for food.
12:34But evidence from Gibraltar is helping to paint a different picture.
12:38The things that are coming out of Gorham's cave are remarkable. A lot of evidence of how sophisticated these Neanderthals were.
12:45You find the stone tools, which were difficult to make. You have the bones of the animals that they were catching.
12:52And it's not just the large mammals that people thought. They're catching seals, dolphins, birds.
12:59But it all tells us that they were able to plan their lives, to plan where they were going to hunt, what they were going to hunt,
13:07and even the fact that they were cooking the food that they were eating.
13:10In 2014, a further discovery at Gorham's cave provided even more compelling proof of just how advanced Neanderthals really were.
13:23This is one of the oddest things that we've ever found in Gorham's cave.
13:28It's obviously a replica that we've got here, and we all call it now the hashtag.
13:32And it's really because it's a series of lines criss-crossing each other.
13:35Analyzing sediment found around the hashtag told the team it must have been made by a Neanderthal around 40,000 years ago.
13:45But these markings aren't idle scratches.
13:49We made it ourselves on similar rock with similar tools, and it took us a long time to do it.
13:56That took 60 strokes.
13:58So the person who did this not only had a skilled hand, they actually were doing this deliberately.
14:08For over a century, archaeologists have been mesmerized by Stone Age murals.
14:14Caves were the backdrop for the first human art, ideas and thoughts.
14:19You have a cave, you have a very nice large canvas. It's quite practical if you want to make a large image.
14:25But beyond that, when you enter a cave, you're passing into a very different world.
14:30People are clearly seeing them as a different space. They're doing something different to their everyday living.
14:36The cave art that we have in Europe, it's not the oldest in the world, but it is absolutely spectacular.
14:42The finesse that you see in the art is really impressive.
14:48But among the colorful paintings of animals are often small, overlooked markings.
14:54Not only hand prints and stencils, but curious symbols, lines and dots.
15:00Found all over the world, some have even suggested that these could be the origins of written language.
15:07Some of the particular symbols may have had a really specific meaning that would be widely understood.
15:16Rather than being a word, it might have been something like an idea, but that people in that whole region would recognize that.
15:23But some markings, found in caves in Spain, appeared to date from much earlier than the known arrival of Homo sapiens into the region.
15:32Giving rise to debates that they might have been done by Neanderthals.
15:37The Gorham's cave engraving is one of the most sophisticated cave markings ever found.
15:43And with compelling evidence of Neanderthal manufacture, it is revolutionizing ideas about their cognitive abilities.
15:51Some people have said it's a map, it's a symbol of the clan.
15:55Some people have said we all have it, it's in the palm of our hands.
15:58I don't know.
15:59But what it does show was that they were capable of abstracting from their world and symbolizing in some way.
16:08They were really humans.
16:10Leaving this symbol at Gorham's cave suggests it held special importance to the Neanderthals, and with good reason.
16:18Because during the last ice age, Gorham's wasn't on the coast at all.
16:25When much water was frozen in massive ice sheets, sea levels in this part of the Mediterranean were over 400 feet lower than they are today.
16:35A fertile plain stretched out from Gibraltar towards Africa, and Gorham's cave would have provided a safe vantage point for the Neanderthals to watch game and plan hunts.
16:51The artifacts found in Gorham's cave are already starting to rewrite Stone Age history.
17:11Could Rue's scan do the same for the ancient Greeks?
17:14Stuart wants to understand the significance of the passage at the back of the cave, near where the Medusa mask was found.
17:26But if the team wants to scan inside it, they're going to have to crawl.
17:33One of the great things about caving, unless you go down there, you can't tell what there is down there.
17:39Unless you squeeze through that little squeeze and stick your head around the corner, you've never seen it.
17:44You've got no idea, there's no way of telling.
17:48That's absolutely just a flaw that is.
17:53But every caver must listen to their instincts when it's time to turn back.
17:57John!
18:03You know I said it was snug, it really is rather snug.
18:06I think I'm actually stuck.
18:08One done.
18:10Now the other one.
18:12With Rue unable to fit through the tight entrance to the passage, he sends in assistant John instead.
18:19All right, yeah, be clear.
18:21OK, I'm OK now.
18:26Once through the tight squeeze, the cave opens up enough to let John set up the scanner.
18:31But working in this section of cave won't be easy.
18:38There's lots of little squeezes to get through.
18:43So many shadows that the laser won't see in particular positions, which means we're going to have to scan every meter or so.
18:51It will take another two days to scan the whole of Gorham's complex chambers.
18:57And this is not the only cave system of its kind here.
19:01Nicknamed the Hollow Mountain by the Romans, the rock of Gibraltar is cut through by many labyrinths of natural caves.
19:12While some have already been mapped and recorded, there may be more inside.
19:18Just waiting to be discovered.
19:20It's this exciting potential that's given rise to Gibraltar's own dedicated cave exploration team.
19:28But they're not who you'd expect.
19:31The official caving unit also happens to include the local firefighters, whose expertise in challenging environments is a valuable resource inside caves.
19:42For the past 10 years, their mission has been to uncover Gibraltar's hidden underworlds.
19:53Since they started exploring, the team have found around 50 new caves, taking the total to more than 200.
20:01The team are starting today's mission in the middle of the peninsula, at St. Michael's Cave, one of Gibraltar's main tourist sites.
20:16Popular since Victorian times, when it became an impressive venue for parties and picnics.
20:28Nowadays, the show caves natural acoustics make it an ideal concert venue.
20:35For us in the caving units, it's lovely to see these bigger caves like this.
20:40We're after discovering some parts of the underground Gibraltar that hasn't been discovered yet.
20:51To hunt for uncharted passages, the caving team must journey deep inside the rock,
20:56descending perilous slopes into the belly of the mountain.
21:01But finding completely new caves in a place as small as Gibraltar is no mean feat.
21:09Many caves were discovered in the 19th century, after Gibraltar was a strategic base for the British Navy during the Napoleonic Wars.
21:17And just like the modern team, many officers stationed here became pioneer cave explorers.
21:27In the 19th century in particular, there wasn't a lot going on in terms of war.
21:31There was no war effort at the time.
21:33But there were a lot of military personnel stationed in Gibraltar.
21:36Now they were getting bored, they were trying to find ways of using their spare time.
21:40And cave exploration was definitely something that they got into.
21:46A remarkable amount of historic graffiti speaks to the volume of people who have set foot in here over time.
21:53But while many have visited these caverns, it wasn't until the caving unit carried out a systematic exploration in 2012 that they were comprehensively mapped.
22:05Until then, myths circulated that this cave was a bottomless pit or even a secret tunnel from Africa through which Gibraltar's famous monkeys, the Barbary macaques, had migrated.
22:18The team discovered that while not endless, the cave was of substantial depth, uncovering two completely uncharted passages running deep underground.
22:28But some of Gibraltar's unexplored caverns cannot be reached by normal caving alone.
22:40Lying on the west of Gibraltar is the entrance to another enigmatic underworld.
22:46With an intricate system of passageways stretching up into the rock, ragged staff is the largest known cave system in Gibraltar.
22:55But much of it has never been explored.
22:58Stewart's mother, Geraldine Finlayson, a prominent scientist here, has long been fascinated by ragged staff.
23:08Discovered by accident at the turn of the 20th century during construction of a man-made tunnel, these caves were once entirely cut off from the outside world.
23:19Geraldine wants to understand more about their unique environment and how they were formed.
23:31She has enlisted the help of world-class cave diver John Valanthan to carry out a detailed survey of one particular flooded cave.
23:40A spectacular but dangerous underwater cavern Rue's laser scanner can't reach, which she's sure holds some of the answers.
23:52So, this is Silent Pool. You can see why it's called that. It's so quiet.
24:06We're really interested in finding out what is inhabiting the water in the cave, so we're really interested in these microscopic animals called copepods.
24:14Copepods are tiny shrimp-like crustaceans which occupy almost every fresh water and salt water habitat on Earth.
24:26With Silent Pool isolated from the outside world for millennia, some unique species of copepod have been found to exist here.
24:35Geraldine wants John to leave some baited traps in the deepest part of the cave in the hope of discovering even more new species.
24:42But, invisible to the naked eye, catching them will be a challenge.
24:49Geraldine's also hoping the dive will reveal clues about the cave's creation deep within the mountain.
24:57The water is lovely and clear. It's not just silent, it's fantastically clear, which is a real treat for someone who's used to diving in Britain, where it's quite often very murky.
25:06But Silent Pool is not for the faint-hearted. Extending for at least 125 feet, the cavern slopes at an angle all the way to the bottom.
25:20If something went wrong during a dive, there would be no quick escape back to the surface.
25:25It can potentially be very dangerous. And in fact, there has been a fatality there.
25:32But John has dived in some of the world's most treacherous underwater environments.
25:37Including in 2018, the Tam Luang cave system in Thailand, where he helped to rescue 12 boys and their football coach from a flooded cave.
25:46Cave divers contend with a number of risks. You can't simply ascend to the surface because you have that rock ceiling above your head.
25:55It's also very easy to become disorientated. It's usually quite binary. You either survive or you don't.
26:02As John's survey dive to the end of Silent Pool may likely take a couple of hours, he's taking several gas tanks and a minimum of three lights to ensure he has plenty of supplies.
26:16A rebreather, which absorbs carbon dioxide, will avoid disturbing sediment and decreasing visibility.
26:36As he sets off into the abyss, the crystal clear waters of the cave allow him perfect underwater vision.
26:43In an otherworldly landscape.
26:51Following lines laid by previous divers, John begins the long, slow descent down.
27:00And it's not long before he comes across something surprising.
27:05A huge limestone column created by a stalactite and stalagmite merged together.
27:11This is a significant clue.
27:14Shaped over centuries by dripping water, for such formations to exist, the cave must once have been much drier.
27:28Gibraltar's sea levels have fluctuated throughout history.
27:31Many sea caves here, like Gorham's, were shaped by crashing waves carving out weaknesses in the rock.
27:42Curiously, similar ones found high up on the cliff sides of Gibraltar are also marine caves.
27:48Sea level has gone up and down over time, but the land in Gibraltar has steadily risen out of the sea.
27:57So it's kind of like bathtub rings, you know, you get the sea level and it creates this level of caves.
28:02But then the land gets risen up and then the next time the sea level gets there, it forms another level, but it's lower down.
28:07Silent pool is different.
28:10Isolated within the rock of Gibraltar, cut off from the wave action of the sea, it must have been formed by an altogether different process.
28:20It's a solution cave, in other words, it's formed as the fractures in the rock allow water to percolate through and over thousands of years that water dissolves bits of the rock and starts forming these cavities.
28:40Weakly acidic rainwater eventually carves out large passageways and cave systems through the limestone.
28:47Today, most of Raggedstaff Cave is dry.
28:52The rainwater which created it must long ago have found its way out of the rock.
28:58So why is Silent Pool now full of water?
29:04At around 50 feet down, John finds another clue.
29:08Something truly remarkable.
29:11As you descend, the fresh water that sits at the top of the cave,
29:15it becomes first brackish and then it becomes salt and there's a mixing zone where this happens.
29:26Known as a halocline, this phenomenon can occur when waters of different densities are brought together.
29:33Salt water sinks to the bottom, while fresh water with a lower density sits on top, forming a sort of cap.
29:41Sometimes when you're diving through the halocline, what you're going through is an area that's almost as if it's shimmery.
29:48You're not really sure whether it's your eyes that are out of focus.
29:52It's actually the different densities that are shimmering.
29:54Even though Silent Pool is around 1300 feet inside the rock, there is seawater at the bottom of this cave, preventing the rainwater from running out.
30:06Geraldine wants to know how the seawater is getting into this apparently landlocked cave and hopes John's dive survey might help.
30:15On the cave floor, John is setting copepod traps. They'll be collected in a few days' time.
30:23Now, he dives deeper in search of an entry point for the sea.
30:27But he soon finds his path blocked by boulders.
30:33It can only mean one thing. This is not a simple cavern, but a huge passage with a collapsed roof.
30:42John searches a few small openings, looking for a route through, but to no avail.
30:48Eventually, he makes his way back up to the surface.
30:55Did you notice whether there was any visible entry of salt water into the cave?
31:02The large passage that this cross rift has collapsed and filled it.
31:07There's no question in my mind that the cave continues in that direction.
31:11But it's just blocked by the collapse.
31:13John believes that the collapsed cave passage could be a direct route in for the seawater, though there is another possibility.
31:23As limestone is porous, the salt water could be seeping through tiny cracks in the Great Rock itself.
31:30But even with today's most sophisticated underwater scanners, there's no way to prove this.
31:36Geraldine will have to wait for future technologies to determine the exact route in of the seawater.
31:50After three days of scanning, Rue finally has all the data from Gorham's Cave to show Stuart.
31:57OK, Stuart, you ready?
31:58I'm ready.
31:59God, I hope you like it.
32:00I hope you like it.
32:02So here we are.
32:03This is Gorham's Cave.
32:05That is spectacular.
32:07It's like a photo.
32:09You think so?
32:10Almost.
32:12Finally revealing this dramatic underworld in all its glory, Rue's data has captured every detail of the main chamber.
32:23Every nook and cranny, every static type has been pinpointed by the precision scan data.
32:28From the 60 foot high bed of deposits at the front of the cave to the metal staircase running to the back.
32:36It's got color.
32:37Yes.
32:38Didn't expect that.
32:40Stuart's most keen to see what lies beyond the tiny opening at the back of the cave, where his team found the mask of the Medusa.
32:47We've been working in this area for quite a long time now, and we've been trying to understand the logistics and plan how to get into the back part of the cave.
33:00The full extent of the area beyond the tight passage can now be seen.
33:07The data reveals a labyrinth of tiny tunnels and chambers, measuring nearly 50 feet across at its widest point and a staggering 121 feet long.
33:19It's amazing to be able to see it like that.
33:22It is actually, isn't it?
33:23It's really quite thrilling.
33:24I mean, you can see this section is pretty much as wide as the entrance section.
33:29Yeah, absolutely.
33:31For Stuart's team, this scan is a valuable resource, allowing them to plan new excavation strategies inside this intriguing space.
33:39In fact, the Medusa's mask isn't the only artifact found.
33:46Ancient pottery, rings, and other items were also found around the tight entrance to the passage.
33:54Not least, the largest collection of scarab beetles found anywhere in the Iberian Peninsula.
34:01What that's showing us is that it's basically one of the really big temples for the ancient Greeks.
34:07Yeah.
34:08The mariners.
34:09It's a place of amazing significance for them.
34:13Greek mariners were treating this cave like a shrine, adorning the opening at the back with votive offerings, and Stuart is keen to know why.
34:23Incredibly, Rue's scan might offer an answer.
34:27Telltale markings running along the wall of the main cave.
34:32That's basically an old dune level.
34:33Remarkable as it may seem, 2,600 years ago, around the time the mask was placed here,
34:41Gorham's cave would have been filled almost to the brim by a massive sand dune, long since eroded.
34:47Evidence from the scan allows this dune to be reconstructed in 3D for the first time, revealing exactly how the ancient Greeks experienced Gorham's cave.
34:59They would have walked up this sand dune and then descended down into the cave.
35:04Yeah.
35:06To their eyes, the dune would have made the cave seem like a dark, narrow road leading down into the bowels of the earth.
35:13Could the ancient Greeks have believed this was the entrance to Hades?
35:20The shadowy underworld where the souls of the dead rested for eternity?
35:26Believing the narrow passage to be a gateway to another realm, it's possible the Greeks may have left their votive offerings here to keep at bay the spirits which lurked beyond.
35:36And they may have had good reason.
35:40To them, the rock of Gibraltar was one of the two pillars of Hercules, legendary monoliths believed to mark the edge of the known world.
35:54Rather than a warning not to enter the cave, these offerings may have been left to appease the gods of the underworld and to ask for protection as the mariners navigated these perilous straits.
36:08This was the end of that encircled sea and where you felt relatively safe.
36:15If you went beyond the pillars, it was the end of the world.
36:19You didn't really know what you were going to find out there in what we call today the Atlantic.
36:24To the ancient Greeks, Gorham's cave was a shrine far from home, marking the edge of their known world and perhaps even a portal to another realm.
36:34And Drew's data has one final secret to reveal.
36:40The scan suggests the cave beyond the tight entrance could be a treasure trove of new finds.
36:47When you go into these lower passages, these very small passages, I mean you've definitely got the impression from these scans that they're actually quite big passages.
36:55They're just full of infill, which of course are covering up more artifacts potentially.
36:59Absolutely.
37:00This is vital new evidence, encouraging Stuart's plans for future excavations inside these passages.
37:07You've got a lot of shifting to do.
37:10A lot of shifting and this is going to help us plan many, many years worth of work.
37:14Yeah.
37:15It's not only Gibraltar's natural caves like Gorham's that people have made use of over the centuries.
37:35The rock also contains plenty of man-made tunnels.
37:46In fact, there are more inside it than the combined length of all the roads on the outside.
37:52Geraldine is taking Rue to a secret location near the top of the Great Rock to see a mysterious man-made underworld that lay undiscovered for decades.
38:09There's just something I want to show you.
38:13Oh, cool.
38:15This is something here that if you didn't know it was here, you could miss it.
38:20Okay, neat.
38:22I'm sorry, it's a bit of a squeeze.
38:24Oh, wow.
38:26These chambers were only discovered in 1997, found by some explorers who felt a mysterious breeze coming from the side of the main tunnel.
38:36So the corridor opens up quite a bit, but have a look at this.
38:47Gosh, it's a room.
38:49Fully plastered, with beams in the ceiling, this secret room presented a real puzzle.
38:55There were no written records of this space anywhere to be found, and the entrance was entirely bricked up.
39:01But it wasn't only being seen that the builders of this room were trying to avoid.
39:08So if you have a look at the floor, these are all cork tiles. Now cork is really good at dampening the noise.
39:16Yeah, yeah, yeah.
39:17So if anything was going on in here, nobody would have overheard it.
39:20Somebody didn't want to be discovered.
39:24Oh, toilets.
39:25Clearly a 20th century structure, this complex is a warren of staircases, rooms, and corridors, lit only by a few narrow slits to the outside world.
39:38Daylight.
39:39That's right, Beth.
39:40Geraldine is in the process of uncovering this cave's intriguing history.
39:51She's sure it served a military purpose, and hopes a detailed scan might help to clarify where the complex sits within the rock, and why it was built here.
40:01I think one of the things it will do is it will show you the structures in relationship to the rock itself.
40:09So that's going to be quite interesting to reveal.
40:14John and Drew immediately get to work.
40:18It's nothing like any other cave I've ever scanned.
40:21It's small, it's intricate, it's a bit dirty.
40:24It's a real thrill to potentially be able to shed some new light on something that's very real.
40:35With its strategic location at the mouth of the Mediterranean, Gibraltar has had a long military history, and battles have been fought and won from its hidden underground.
40:46If you control Gibraltar, you control a lot of the maritime traffic coming through.
40:50So just about any power that's wanted to exert some influence in that region has wanted Gibraltar.
40:57The vast majority of tunnels were blasted out during the Second World War, when Gibraltar served as a base for British naval power in both the Atlantic and Mediterranean.
41:08Over 18 miles of World War II tunnels still run beneath the Great Rock, some even large enough for lorries to pass through.
41:21The tunnel we're in now is called the Great North Road, and it's the backbone of the main tunnel system.
41:27Dough within just three years, this road was part of a huge underground warren, stretching the entire length of the rock, and provided a safe arena for all types of wartime activity.
41:40But a complex this size needed an ingenious navigation system.
41:43So that the soldiers would be able to find the way, they treated it as if it was the main road going up from the south to the north of England.
41:52So we've just come past Peterborough Chambers, and as we head in further north, we'll end up all the way at Durham.
41:59Built to house 17,000 troops, this was a complete underground city, with all the facilities needed to survive a long bombardment.
42:09It had several large kitchens.
42:12These are the ovens, 50 gallon parts.
42:16A bakery, offices, and living spaces, all built inside secondary structures, protecting them from water seeping through the rock.
42:27As the base of the British Army Brigade Headquarters, the tunnels also provided a secure space to plan tactics.
42:35The complex even included a hospital.
42:38It's had its own operating theatre, it's had a ward, and it's had a special ventilation system to keep all the people healthy in there.
42:46In a territory of only three square miles, taken up mostly by the Great Rock, building within it was an efficient solution.
42:54So could Geraldine's puzzling cave simply be a similar wartime facility?
42:59With the scan complete, and all the data processed, Rue can now see its full extent.
43:10I have to say, it's really quite amazing.
43:15It's certainly complex, and I can understand why she wanted it mapped.
43:20The data reveals an isolated pocket of rooms and tunnels, with no way in except for the bricked-up entrance, and tiny slit windows the only access to the outside world.
43:35It's nothing like the rest of the man-made tunnels.
43:39This must be a secret lair, designed for a very specific purpose.
43:44In 1941, the British caught wind of a Nazi plan to capture Gibraltar, and use it as a stepping stone to invade North Africa.
43:53With the outcome of the war hanging in the balance, the British came up with a plan, Operation Tracer.
44:04In case Gibraltar should fall into enemy hands, a small team of men were enlisted to stay behind and gather vital intelligence.
44:13A secret spy complex was built inside the rock, but with all records classified, its very existence was unknown.
44:23For a long time there had been rumors, people had talked about, but you know, nobody really takes them very seriously.
44:30Lying untouched like a time capsule, it wasn't until 50 years later that the cave's exact whereabouts were revealed.
44:39With the discovery, new witnesses involved in Operation Tracer started coming forward, their testimonies the final pieces of a puzzle half a century old.
44:50If the Germans took Gibraltar, I was to disappear up to the cave at the top of the rock and lock myself in.
45:00Finally confirmed, this is Stay Behind Cave.
45:06This is Stay Behind Cave.
45:09Placing the terrain data on top of the cave scan reveals exactly how this spy layer worked.
45:15Skillfully chiseled into the very top of the rock, the scan reveals how two observation points, one facing east towards the Mediterranean, the other west over the Bay of Gibraltar, provided safe vantage positions to see enemy troops arriving from either direction.
45:31And from a central radio control room, they could communicate any important information back home.
45:40Designed to accommodate six men, Stay Behind Cave was to be entirely closed off from the outside world.
45:46The plan was for these soldiers to remain here for up to a year, cataloguing the Nazi Navy's every move so British headquarters could plot counterattacks.
45:59The cave had to be entirely self-sufficient.
46:02Waste would have been macerated and secreted into cracks in the mountain, and drinking water collected from rain deep inside the rock.
46:11A 10,000 gallon water tank was built, and you can see it here.
46:18And the radio control room provided more than just a way of transmitting messages.
46:24It was also the cave's powerhouse.
46:28If you look over here, you can see what looks like the fan from a radiator of a car, and that's exactly what it is.
46:35But it was attached to a bicycle. That would have been used to generate the electricity to power the radio.
46:43Spying was dangerous work. Messages could only be sent at night as the radio aerial had to be extended out of the top of the rock.
46:51And the scan also shows just how close Stay Behind Cave was to other military passages.
47:00Only around 24 feet away from storage chambers, the lair was truly hidden in plain sight.
47:07I think that's the whole trick of the way that this space works.
47:10Hidden in this tiny little corner where no one would expect it to be is this entirely secret complex.
47:15If the men had been found by the Nazis, they would almost certainly have been shot.
47:23It's a story fit for a spy thriller, and may have actually inspired one.
47:29It was absolutely top, top secret. Only three people were present in the first few meetings.
47:36One of the people present was Ian Fleming.
47:38Famous for the James Bond spy novel series, Fleming was himself a naval intelligence officer.
47:47Involved with several covert operations in Gibraltar, he may well have drawn inspiration for his later writing from this very operation.
47:55We think that a lot of the ideas that he incorporates into the novels, of course, they're modified, but the seeds of those ideas came from this Operation Tracer.
48:06In the end, the Nazis never did invade Gibraltar.
48:11With the Allied Victory, Operation Tracer was abandoned.
48:15The cave was sealed, and in case it should ever be needed, all who knew of it were sworn to secrecy.
48:22When I walk around here, I know that these rooms were never actually used, but it's a bit like how Howard Carter must have felt when he first looked into Tutankhamen's tomb.
48:34The fact that people spent so much time and so much effort here, but nobody divulged anything.
48:43The discovery of Stay Behind Cave has even prompted new theories that it may not be alone.
48:50There may be other secret spy layers here just waiting to be found.
48:54The whole story just makes you want to get your caving helmet on, get your light on, get your sensing equipment on and plow through those tunnels and look for this potential other place, which may or may not exist.
49:06Man-made underworlds have come a long way since World War II.
49:17Today, there are over 10,000 underground military installations around the world.
49:22Some, like the Cheyenne Mountain Complex in Colorado, are cutting-edge military cave structures.
49:31Built at the height of the Cold War as a nerve center for nuclear missile defense, it remains one of the U.S.'s most secure surveillance bases.
49:40Located under almost 2,000 feet of granite, with 25-ton blast doors, this huge complex can resist almost any type of attack.
49:51It's a far cry from Gibraltar's wartime tunnels, but it shows just how vital caves remain for our protection.
50:02We've lived underground for, you know, hundreds and hundreds of thousands of years.
50:06And the fact that even though we have 21st century technologies and sort of surveillance systems, but actually we go back to the caves to try and protect those is a really interesting image.
50:15With both scans and dive now complete, Rue and the team have accomplished their mission in Gibraltar.
50:29Finally revealing why the ancient Greeks venerated Gorim's cave.
50:34How Stay Behind's unique layout in the top of the rock would have proved vital in the event of an enemy invasion.
50:41And although analysis of the traps did not reveal new species of copepod, their presence in the silent pool suggests there must be a wider ecosystem just waiting to be discovered.
50:54So how have you found your time here, Rue?
50:56Oh, it's been fascinating. Let's be honest, it's not on the world caving map, is it?
51:02They're not the biggest caves, they're not the deepest caves, they're not the longest caves, but they're certainly some of the most interesting, absolutely.
51:07Gibraltar's underworld tells us the story of over 120,000 years of cave life.
51:18What more secrets will these caves reveal in the future?
51:22Only time will tell.
51:37p.m.
51:59P.m.
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