00:00Humans invest a lot of time and money exploring the unknown.
00:05First, outer space.
00:06Then, the ocean floor.
00:08But there's another frontier we're trying to conquer.
00:11Scientists call it the deep frontier.
00:14I'm talking about how far down to the Earth's core we've made it so far.
00:19During the 60s, humans tried digging the deepest man-made hole on Earth.
00:24This took us deep into the Arctic Circle.
00:26What later became known as the super-deep borehole,
00:30is also the deepest artificial point on Earth,
00:33reaching the depth of 40,230 feet.
00:36It took 20 years to drill this far into the Earth's crust.
00:40And researchers say this is only about one-third of the way through to the mantle.
00:46Here's a quick anatomy of our planet.
00:48If the Earth is like an onion, the crust is like the thick skin of this onion.
00:53This part is only 25 miles thick.
00:56Beyond this is the 1,800-mile-deep mantle.
01:01And after that, right at the center of the Earth is the core,
01:05which is made up of two layers, inner and outer.
01:08Now, can you guess where on the planet the crust is at its thinnest?
01:12On the ocean floor.
01:14When American scientists understood that,
01:17they started drilling holes on the seabed.
01:19This gave way to Project Moholl.
01:22It began in 1961,
01:24but their technology to drill deep holes on the ocean was really bad.
01:29They had to improvise.
01:30They installed a system of propellers along the sides of their drill ship
01:34to keep it steady over the hole.
01:37The main challenge was to drill as vertically as possible.
01:41Otherwise, that deep hole would look more like a deep maze.
01:44Both the Kola Superdeep Borehole Project and the Project Moholl
01:48came to an official stop in the years that followed.
01:52But hey, humans have built some pretty deep stuff underground.
01:56In 1963, a Turkish man took a swing with his sledgehammer
02:01to improve his basement and discovered a tunnel.
02:04That tunnel entrance led to more openings
02:06that connected a multitude of halls and chambers.
02:10He didn't know it, but he had discovered Derengoyu.
02:13This once-lost city was up to 18 stories tall and 200 feet deep.
02:19The city was large enough to host 20,000 people,
02:22and it was simply standing beneath Cappadocia in Turkey the whole time.
02:28Cappadocia hosts hundreds of subterranean dwellings.
02:3140 of these underground cities are at least 2 stories high,
02:35but none of them are as huge as Derengoyu.
02:38There's not a lot of historical accounts of how Derengoyu was born.
02:42Some speculate that the oldest part of the complex
02:45was dug about 2,000 BCE by the Hittites.
02:49The Hittites were an Indo-European civilization
02:52who formed one of the first major civilizations
02:55of the Bronze Age in West Asia.
02:58Scholars think they built this underground city
03:00to protect themselves from invasion.
03:03Others think it was actually the Christians
03:05who built the city in the first century CE.
03:07They speculate that it could have been constructed
03:10to protect people from the intense temperature variations,
03:14extreme heat in summer and freezing cold in winter.
03:19Geographically, the terrain under Cappadocia is great for building.
03:23The rocks are soft, which makes tunneling easier.
03:27For that same reason, opening large caves underground is fragile.
03:31That's why most chambers have pillars that support them.
03:35On the bright side, and we are,
03:37none of the floors at Derengoyu have ever collapsed.
03:40The builders of Derengoyu thought of everything.
03:45The city was shut off from the world by huge rocks that hit its entrance.
03:49It was ventilated by a total of more than 15,000 shafts,
03:54around 10 centimeters wide.
03:55This way, they guaranteed enough breathing air in the upper levels
03:59in a way that they ended up being used as the living and sleeping quarters.
04:04The lower levels of this impressive maze were mainly used for storage.
04:09But honestly, they had it all.
04:12A room for a wine press, for domestic animals,
04:15small religious buildings.
04:17They certainly didn't spare any efforts
04:19to make that underground city as complete as an above-ground one.
04:23Now, speaking of impressive underground stuff,
04:28Switzerland has the longest tunnel in the world,
04:31built here on the Swiss Alps.
04:33This absurdly huge tunnel was built over 8,000 feet below the surface.
04:39Check this out.
04:40This is what the Gothard Base Tunnel looks like.
04:43The first idea to build the tunnel was put forward as early as 1947,
04:48but construction only officially began in 1999.
04:51It took them years to approve the project
04:54and then scout for the perfect location.
04:58Even so, the 73 different rock types that compose Massif
05:02turned the construction into a real challenge.
05:05Some of the rocks were as hard as granite
05:07and others were as soft as sandstone.
05:10This meant they needed different approaches and equipment
05:13so the whole thing wouldn't crumble down.
05:15Overall, they had to excavate about 28 million metric tons of rock
05:21to make way for the tunnel.
05:23There were many other challenges to build that far into the Earth.
05:27The deeper into the mountain they got,
05:29the hotter the temperatures were.
05:32Since there's no natural ventilation,
05:34temperatures would get as high as 115 degrees Fahrenheit,
05:38making it really hard for workers to endure.
05:40The tunnel stretches for 35 miles from one end to the other.
05:47During the construction, workers had to use four massive tunnel boring machines,
05:52aka moles, to penetrate through the massive.
05:55Each of these moles was around 1,400 feet long.
05:59But that was one of the best things they could do,
06:02since these machines limit the disturbance to the surrounding ground
06:06and helped to produce a smooth tunnel wall.
06:10You'd never guess that the world's deepest basement
06:13belongs to the Sydney Opera House in Australia.
06:16The building goes as deep underground as it grows above ground.
06:20But this basement isn't just the Opera House's foundation.
06:24It's a parking lot.
06:26This unique parking garage penetrates 120 feet underground,
06:31or 12 stories deep,
06:32while most car parks are only 4 or 5 stories deep.
06:36But that's not all that makes it unique.
06:39You see, if they decided to build a normal rectangular parking space,
06:43this would end up eliminating a bunch of possible parking spaces.
06:47And since they wanted to maximize the space they had available,
06:51they had to come up with some superb structure.
06:54First, they opened up a man-made cavern to make that dream possible.
06:59Then they had to come up with a design.
07:01That's what led to the donut-shaped cavern,
07:04or what engineers call a double helix.
07:07This design shocked the engineering community for its innovative approach.
07:12But hey, it worked!
07:13Today, there is space for over a thousand cars down there,
07:17which seems about right for an Opera House.
07:21Let's wow once again real quick.
07:24Some underground structures may take up to an hour to reach.
07:27Take this gold mine in South Africa that lies around two and a half miles below the surface.
07:34Yeah, you may think, hey, it's just a couple of miles,
07:37why does it take an hour to get there?
07:39But the conditions are hard, and it's not for the weak.
07:43But Milan Ladina, an Ecuadorian runner,
07:46even completed a half marathon there,
07:49despite temperatures reaching 104 degrees Fahrenheit,
07:52high humidity at 80%,
07:54and an atmospheric pressure of 1.4,
07:57comparable to what divers experience 5 meters under the sea.
08:01Yikes!
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