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00:02The great gothic cathedrals of Europe.
00:06Soaring medieval skyscrapers that hold age-old secrets.
00:13How did their creators build so high using only simple tools?
00:20What beliefs inspired them to reach for the sky?
00:23It is an incredible building.
00:25The height, the footprint, the presence, it's really spectacular.
00:28And can new technology unlock the secret science that protects these stone giants from disaster?
00:35They had to be afraid that the earth pressure would bring the pits to collapse.
00:42The only way to solve these mysteries is to blow apart these mighty medieval megastructures stone by stone.
00:56Diving deep through their walls, vaults and foundations will help us unearth the astonishing engineering secrets at the heart of
01:03these age-old wonders.
01:13The German city of Cologne on the banks of the mighty Rhine River.
01:20This is the home of the world's most ambitious megastructure.
01:27Cologne Cathedral.
01:30A mysterious monument that began life in the Middle Ages.
01:35Seven hundred and fifty years ago, the people of this city embarked on a colossal challenge to build the tallest
01:41cathedral of the day.
01:44Twenty times higher than the wooden buildings in town.
01:51The sheer scale of this church astounds modern experts who try to unlock its secrets.
01:57The amazing thing about the Cologne Cathedral is that if you go back in time, someone actually had the vision
02:03to design and conceive a building of this size.
02:07And it's spectacular.
02:14Cologne Cathedral is a 130,000-ton stone giant.
02:21There's enough glass in the walls to clad a 30-story skyscraper.
02:32Huge stone ribs prop up the ceiling.
02:37As well as the 600-ton lead roof.
02:43Stretching nearly 525 feet into the sky,
02:46two enormous towers once made this church the tallest building on earth.
02:55So how did they pull off this medieval marvel?
03:03Historians know surprisingly little about what actually went on during the early days of Cologne's construction.
03:10Few records have survived.
03:15Now architect Doug Pritchard is rediscovering how the cathedral was built by stripping it apart stone by stone.
03:26Doug's got the perfect tool to do this.
03:29A digital laser scanner.
03:34This machine will allow him to reveal the building in unprecedented forensic detail.
03:39The scanner is really quite incredible.
03:42It can generate approximately a million dimension points per second.
03:48The way that the scanner works is that it does a 360-degree sweep.
03:53It will show things like the depth of the vaults, the size of the columns in a very precise way.
04:00We take all that data together, and the result is you have a 3D map of the cathedral.
04:08Today, Doug's adding another important piece to his puzzle.
04:14Good job.
04:15Good job.
04:16Good job.
04:19He's on his way to the central balcony.
04:24If he can use scanning to expose the cathedral's underlying structure, he can start to dissect how it works.
04:34One of the great things about laser scanning is that you can pull the data apart.
04:37Let's use the scan data to explain the architecture, how the building stands up, how the engineering works.
04:45Doug's scanner fires an invisible laser beam as far away as 614 feet.
04:53It records each time the beam hits something solid and bounces back.
05:00And then turns this data into incredibly detailed images, so that Doug can study the structure from every angle.
05:09In total right now, we're at about 660 scans.
05:13Probably by the end of this week, add in another 20 or 30.
05:19Doug now needs to crunch a huge volume of data.
05:24But he can already start to see the master plan of the people who built this incredible monument.
05:34The two tall towers are visible for miles.
05:40At the top, the spires are open to cut down wind resistance.
05:46And at the bottom, the walls are rock solid to bear the enormous weight of the towers.
05:52But the church behind is completely different.
05:56Under the skin sits a skeleton of slender stone arches.
06:02Incredibly, it looks like the core of a modern skyscraper.
06:06Yet this is the brainchild of medieval master builders.
06:11Men who lived 700 years ago.
06:17So how did Cologne's citizens even begin to imagine that such a tall building could be possible?
06:28200 miles away in northern France, another team hunts for clues at an even older cathedral.
06:38This is Amiens, the blueprint for Cologne.
06:45This soaring church has dominated the city's skyline for nearly 800 years.
06:51It's the tallest complete cathedral in France.
07:00Investigator Guillaume Calron uses laser technology to unlock how Cologne's predecessor was built.
07:08I was born actually in this city, so I know the symbolic aspects of this cathedral.
07:14Guillaume is hoping to find tiny clues that might help reconstruct Amiens' lost original design.
07:20Currently, there is not any actual map of the cathedral.
07:25So what we are doing with this kind of device is to measure it very precisely in order to have
07:31the first real actual map of the cathedral.
07:37Amiens' construction began in 1220.
07:41It's one of a handful of cathedrals close to Paris whose architects dreamed of building closer to God.
07:48They wanted to build the highest and the widest cathedral because at that time, in the Middle Ages,
07:54it was like a race between different cities in France to build the most important cathedral.
08:04Guillaume's scans reveal that the central arch is 131 feet high, but the columns are surprisingly thin.
08:12From this scan preview, we can look at the details that were acquired by the device.
08:17For instance, the columns here in the transept.
08:23But until the 12th century, columns this tall and slender would have been too weak to support such an enormous
08:29building.
08:32So how did French architects solve a puzzle that has baffled even the great builders of ancient Rome?
08:38The key is in the shape of the arches.
08:44In a classical Roman arch, the stones at the top push sideways against the columns.
08:55As you build higher, they start to buckle and the ceiling will collapse.
09:05But French engineers realized that if you make the arch pointed, the forces now flow down towards the strong base
09:13of the column.
09:17Now the same columns can take more weight. The arch can be much taller and the church much bigger.
09:27Amiens, with its pointed arches, showed Cologne's citizens a glimpse of what was possible.
09:38But what drove the Germans to build even higher?
09:43The secret lies at the heart of Cologne Cathedral, inside this golden shrine.
09:51Some believe it holds the bones of the three wise men.
09:57Peter Fusenich is the cathedral's current chief architect.
10:06He oversees an army of more than 70 people who keep this building running.
10:16Today, the church attracts six million visitors a year.
10:22But Peter thinks that in the Middle Ages, it was also this tomb that was the star attraction.
10:31The shrine is, of course, one of the reasons why they built Cologne Cathedral in the first place.
10:37It's less the shrine that was built later.
10:40It's more what's inside it. I'm talking about the bones of the three wise men.
10:45This holy relic arrived in Cologne in 1164 and immediately triggered a huge stream of pilgrims to the city.
10:54That meant that very quickly the old church became too small, which is why they had to build a bigger
10:59cathedral.
11:05This mysterious holy shrine inspired Cologne's founders to start work on a cathedral so big
11:11that they knew they would never see it finished in their lifetimes.
11:16This is something that's still immensely fascinating for us today.
11:21Cologne Cathedral is one of humanity's first ever multi-generational projects.
11:27Something you have to hand down to the next generation and have faith that they will actually carry on building.
11:36These men hoped that their children and grandchildren could overcome the next huge obstacle.
11:43How would they build a church with enough room for 20,000 worshippers made from fragile walls of glass?
11:57Cologne Cathedral is a mysterious medieval skyscraper.
12:02It covers 86,000 square feet.
12:06How did her creators illuminate a church this big with heavenly light?
12:12Hundreds of years before electricity.
12:17The answers are hidden deep within the fabric of the cathedral.
12:26The towers rest on thick stone pillars to stop them from collapsing.
12:34But the main hall of the church is much more open.
12:40There's enough glass here to cover a football field.
12:45The upper walls in the middle are almost entirely glass, just inches thick.
12:54And they're crowned by a massive lead roof.
12:58So how did medieval engineers construct windows so big, so fragile, yet so strong?
13:09In the middle ages, craftsmen had no way to make huge, flat sheets of glass.
13:16Ulrika Brinkman studies the ingenious solution that allowed them to push the boundaries of technology.
13:27Medieval glassmakers divided up each window into panels.
13:32They made them from tiny stained glass fragments held together by lead.
13:37Medieval glass is blown by mouth by glass makers.
13:42They have a large glass pipe.
13:43And so the size of the glass panels that are used for the glass painters is limited.
13:50You see the very, very dark deposits on the glasses.
13:54In this case, luckily, they can be easily removed.
14:02The fragments still look dark after cleaning.
14:06But they come alive in the light.
14:10Light was a material in the medieval understanding.
14:15And how light fords through another material, like glass, without disturbing the glass, was without any explanation.
14:26These exquisite details would have been imperceptible to worshippers on the cathedral floor.
14:33But this kaleidoscope of glass creates the impression of a church reaching towards heaven.
14:40Entering the cathedral in medieval times, it must have been breathtaking.
14:45The sun falling through and enlightening the interior.
14:49This was very impressive.
14:52They are still breathtaking today, and even more in those days when people used to live in small and dark
14:59houses.
15:03But the extreme height of these windows creates a serious problem.
15:11Pointed arches help make taller, more elegant churches with big windows from top to bottom.
15:17But build too high, and the structure collapses.
15:22Supporting walls, called buttresses, could help take the strain.
15:27Only they'd block out light and cut across the aisles.
15:34Medieval engineers discovered that if they build columns on the outside with high beams reaching across,
15:40they could build what are known as flying buttresses.
15:45Now they could install huge windows to bring them closer to God and let in heavenly light.
15:53These flying buttresses help support the walls and roof from the outside.
16:00But what stops the ceiling from crashing down inside the church?
16:04How does it appear to hang in mid-air?
16:14Wolfgang Cooper is one of 20 expert stonemasons at Cologne.
16:19He knows that working with the soft limestone requires complete precision.
16:24With this saw blade I borrowed from the metal workers, I can easily cut into the stone.
16:31The stonemasons are like surgeons who care for a 750-year-old patient.
16:37Today Wolfgang has the challenge of replacing a stone block, called a balderhin.
16:46At the moment I'm working on this finial.
16:48I'll have to carve out these gaps and remove the material in between to bring out these leaves.
16:54Extremely small features.
16:56The original stone that formed part of the wall is damaged beyond repair.
17:02This was damaged in World War II.
17:05The front part of it is completely gone.
17:08All that's left is the back end.
17:12Carving this copy will take Wolfgang a year and a half to finish.
17:19Did Wolfgang's medieval predecessors' meticulous attention to detail contribute to the cathedral's incredible durability?
17:41Stonemasons at Cologne Cathedral are revealing the innovative design of this soaring medieval skyscraper.
17:47Instead of thick stone walls to carry the ceiling, they carved pointed arches and joined them together like a ribcage.
17:56In between they put just a thin layer of bricks that forms a smooth ceiling, called a rib vault.
18:09Slender columns channel the weight of this vault down to the ground.
18:14This way the ceiling doesn't need support, it supports itself.
18:22The forces on this vaulted ceiling are enormous.
18:27And just a few stones out of place can spell disaster.
18:36The older cathedral of Amiens in France reveals the risks of this audacious design.
18:44Laser scanning expert Guillaume Caron is at Amiens to investigate damage to the rib vaults.
18:56We have on the ceiling a big crack that, with our laser scanner, we are able to measure.
19:04This is the scanning result.
19:06And as close as it causes the walls of the entrance.
19:07Guillaume discovers that this crack is about two inches wide.
19:13There is also other cracks inside the main nave of the cathedral.
19:18So on the top of these tall windows.
19:23Guillaume can't say exactly what's causing every crack.
19:27but experts think that Amiens has a serious design flaw outside the flying buttresses are
19:34too high and can't take the strain of the roof so the sides have buckled tearing the rib vaults
19:42apart so is the damage getting worse from our latest results we measure that this crack is
19:50about five centimeters and we compare that with the measures we made four years before actually
19:57there is no increasing of this thickness and this is a good news for the cathedral
20:08in Germany at Cologne Cathedral the ceiling looks perfect but is it
20:17Doug Pritchard ventures inside the roof to investigate he wants to laser scan Cologne's
20:23rib vaults from above right now we're between the crossing and the older part of the building
20:30the choir the way that the scanning is going to work is I'll have the scanner positioned all
20:35the way around the this opening area here and with that data I'll take the data from below
20:42that was scanned earlier and combine the two and with that I'm able to virtually slice through the
20:48cathedral Doug hopes his scans will expose the cathedrals inner workings and show them like
20:57never before I think this is a fantastic image in that it clearly clearly explains the architecture
21:05of the cathedral the finished 3d image captures the cathedral with extreme accuracy and what I focused
21:15on here is where we're currently standing and what's interesting is the rib vaults are actually almost like a thin
21:23membrane but the actual engineering and structure is being picked up by the pointed arches you have the columns
21:32which are taking the weight down to the foundation you also have the flying buttresses which are pressing
21:37inward slightly and so they're all in in harmony in terms of how the structure of the cathedral works
21:45Doug scans present an even more intriguing mystery how did ancient engineers build a rib vault over 131
21:54feet above their heads without it crashing down investigators think that the builders first put up wooden scaffolds all
22:04the way to the top of the arches they stacked stone blocks on top to build the ribs with extreme
22:13precision so
22:16the centerpiece would fit perfectly
22:23then they filled the gaps between the ribs with lightweight bricks and covered them with mortar
22:35finally they remove the wooden scaffold praying that their ceiling would hold
22:46cologne's builders had created a perfectly balanced skyscraper hundreds of years before those of the modern era
22:56but how has this colossal cathedral survived centuries of wind rain and natural disaster
23:10cologne cathedral is a remarkable survivor
23:15this stone giant stands in one of europe's most active earthquake regions
23:25more than 120 quakes have hit the surrounding area over the last 300 years causing moderate
23:32modern buildings to crumble
23:35so what's the secret
23:38how is this mysterious structure survived life in the danger zone
23:51what's the secret of it
23:55clous geeseen is a seismologist at the University of Cologne
23:56Klaus guy insin is a seismologist at the University of Cologne
24:02he leads a team of specialists to the cathedral roof
24:06they've come to retrieve data revealing how the cathedral reacts during an earthquake
24:12We started to monitor the cathedral in 2006.
24:16From time to time we have to do maintenance and to see how the station is working.
24:23Can you give the control F?
24:27These instruments are motion sensors that will pick up even the tiniest movement in the fabric of the church.
24:35On the left on the small screen here we see the ground motion that is just being recorded here at
24:41this moment.
24:42And we can see that it's actually working. I just have to stamp a little bit with my foot.
24:47You see it is rather sensitive. We are a couple of meters away from the sensor but anyway you pick
24:53up the signal.
24:54This equipment is helping Klaus to predict what would happen in a big quake.
24:59We have had several strong earthquakes with magnitudes between 6.5 and 7 in the past 20-30 thousand years
25:07in the area.
25:08And geologically this is a very short time span and such an earthquake could happen any time.
25:15It could hit today or next week.
25:19He fears that a major earthquake could shatter these ceiling vaults.
25:25But Klaus's work reveals that the towers would behave very differently.
25:31Here we see a recording example of a real earthquake.
25:34It was about 90 kilometers away from the cathedral and had a magnitude of 4.5.
25:40The red traces are the ones that were recorded in the basement of the cathedral which basically is the input
25:46signal which comes into the building.
25:48And for comparison the blue signals that we see here, this is the motion of the tower.
25:54We see the tower starts swinging back and forth and this eigenmotion of the tower then lasts for several minutes
26:01that we can record it.
26:03It's an astonishing mystery.
26:06How did Cologne's architects create towers that swing like pendulums instead of crumbling to the ground?
26:21Even with all these slender columns, this building is no featherweight.
26:31Medieval engineers took care of this weight with secret structures hidden underground.
26:38They layered 120,000 tons of rocks into supermassive foundation pillars, stretching deep underground.
26:49Just as much stone lies beneath as soars above.
26:54Balancing this cathedral perfectly.
27:00Cologne's founders protected their cathedral with deep foundations.
27:05But in the middle ages, most people saw earthquakes as a punishment from God.
27:10And thought only prayers could prevent them.
27:14So did the cathedral's builders really understand the true forces of nature at work?
27:26Archaeologist Ruth Stinnisbeck investigates what they knew about the ground they were building on.
27:34Her quest takes her down to an archaeological dig underneath the cathedral floor.
27:42Here, investigators were astonished to discover that this foundation pillar goes down 52 feet.
27:49It's as deep as the pillars that support the Empire State Building.
27:56But what really shocked them is what they found at the bottom.
28:07So why did the medieval diggers risk flooding their own construction site?
28:13Ruth thinks it's because they discovered an even greater danger.
28:19Sand.
28:20The city of Cologne and also the cathedral is just standing on sand and gravel.
28:25And that's the sand we also have down here.
28:28Look, it's nothing solid, just sand.
28:36The builders here worried that the cathedral would sink into Cologne's soft sand.
28:42So they decided to push their foundations as deep as possible to minimize any risk.
28:48It must have been like digging on a beach.
28:52So how did they do it?
28:57If workers tried to excavate a hole wide enough and deep enough in one try, the loose sand would have
29:03caved in and crushed them.
29:09So they dug only in shallow steps, shoring up the sides with wooden boards, stopping just before the water level.
29:19They piled in layers of volcanic rocks and mortar to build up mighty stone pillars over five stories tall.
29:31These formed rock solid foundations, supporting 120,000 tons of stone to this very day.
29:44Archeologists have unearthed the startling remains of this ancient construction site.
29:54Over here we have the foundation of one of the pillars upstairs in the cathedral.
29:59And what we see is a mortar surface.
30:04Ruth believes that these splinters are the remains of the wooden boards the builders used to hold back the sand.
30:12You see one plank starts over here to there.
30:16And the next plank from over here to here.
30:21When I look at the wood I try to imagine how the people must have felt when they worked down
30:27here in those very deep and dark pits.
30:29And yeah, they had to be afraid that the earth pressure would bring the pits to collapse.
30:36And only those planks would prevent it from such a catastrophe.
30:42And so I think when you stand down here, you can feel how the people might have felt at that
30:48time.
30:49Archeologists like Ruth continue to investigate this intriguing underworld.
30:58But there's another surprise above ground.
31:04Behind the beautiful vaulted ceiling lies a strange structure.
31:11What could it be?
31:19Cologne Cathedral is the most ambitious medieval cathedral in the world.
31:25This towering structure conceals many surprising secrets.
31:30There's much more to it than meets the eye.
31:37Under the hood of this stone giant hides a structure that looks out of place.
31:49It's a gigantic framework of slender wrought iron struts that hold up the 600-ton roof.
31:58Until the Eiffel Tower, this was the most advanced iron construction on the planet.
32:09So what is this metal marvel doing inside a medieval building?
32:17Drawings and chronicles record the construction work on the cathedral ground to a halt in 1530.
32:24It didn't start up again for more than 300 years.
32:29So how did 19th century craftsmen know how to finish the building?
32:37Chief architect Peter Fusenich uncovers the answer.
32:42A faded 13th century design for the two towers that was once lost, but rediscovered by chance nearly 600 years
32:50later.
32:51The architects of Cologne Cathedral in the 19th century were very lucky to find that medieval plan to have the
32:59possibility to build the facade so they could finally finish the dreams of the medieval architects.
33:06The sketch, called Plan F, looks like a modern blueprint, but it has no dimensions, no instructions.
33:17This Plan F was not meant as a construction plan.
33:20It was a plan to convince those who financed this cathedral, of course.
33:29So how good a job did the 19th century builders do?
33:34Stone Restorer Sophie Hupner belongs to an army of experts who uncover how the final builders had to make compromises.
33:47Sophie and her colleagues are on their way to fix a problem with the flying buttresses built in the 19th
33:53century.
33:55The work of a stone restorer is maybe comparable to a medicine, a doctor.
34:02The medieval builders preferred to use a hard stone called trachyte.
34:06But the rock in these flying buttresses from the 19th century is soft sandstone.
34:12And it's already starting to crumble.
34:18What we can see here is some deteriorated area.
34:22Here we have all the rain is coming from above and just washing.
34:28Here you see it's much drier and there is the collection of dirt.
34:35It gets black because of the pollution in the air.
34:40And these parts where the stone is like sand, it gets washed down.
34:45And that's why it's much clearer than the rest of the stone.
34:50Sophie needs to know how bad the damage is.
34:55So her first task is to test how the stone reacts to rain.
35:01If you put this drop on, you see the stone is like a sponge.
35:06During the rain, the water goes into the pores of the stone.
35:10And when the stone dries, every material that was in the water goes out and is staying on the surface.
35:23Sophie can treat some of the sandstone with a special waterproofing chemical.
35:30On this surface, there's a hydrophobic agent.
35:34So when I put a water drop, you can see it stays.
35:41But some stones can be rotten to the core with no visible signs on the outside.
35:50So the team decides to probe the inside of the buttresses with a special drill hooked up to a computer.
36:00Sophie's colleague Jasper drills into the soft sandstone.
36:10The computer measures the resistance to the drill, revealing weaknesses in the stone.
36:16What will the results tell us?
36:23At the Cologne Cathedral, a restoration team must repair and replace this buttress.
36:28Not even stone lasts forever.
36:30We will use some consolidant agent to make the stone hard again.
36:36And then we test whether it worked.
36:39So why did modern builders use such poor quality stone?
36:46The problem was they couldn't find enough good material to finish such an enormous task.
36:56When they started building, all that existed was the back end of the church,
37:00the ruin of the south tower, and a huge half-filled gap in the middle.
37:07Just to get this far, medieval builders had ripped the top off a nearby mountain.
37:13But this source of stone ran dry.
37:18So 19th century builders had to find thousands of tons of sandstone from many different sources all over Germany.
37:28Here, they used the highest scaffold of the time to place the stones.
37:33And finish the cathedral in just 38 years.
37:41So did the builders sacrifice quality in their race to finish the cathedral?
37:48Local legend says there's something not quite right about the towers.
37:54Architect Doug Pritchard is going to use his scanner to measure just how tall they really are.
37:59There is a certain mystery regarding the heights of the two towers.
38:03Are they exactly the same? Is one higher than the other?
38:06And that's what the scanning will confirm.
38:11Doug hitches a ride through the core of the north tower.
38:21We're up on the north tower at the 100 meter level.
38:25And what I'm going to do is set up the tripod and scanner inside so that we can capture the
38:31entire interior of the tower.
38:35This is the first time that anyone has measured the towers with such accuracy.
38:41What I'm able to do is do a 360 degree scan of the entire interior of the tower.
38:48And that will give me sub-centimeter level data.
38:51The scanner can generate over 1 million dimension points in a second.
38:56So within 15 minutes we're going to have a massive amount of information.
39:02Doug puts the scanner to work.
39:07The final stone was laid on top of the south tower in 1880.
39:11Construction had lasted 632 years and 2 months.
39:18Doug makes a discovery that astonishes even him.
39:23Looking at the data, comparing the height of the south tower and the north tower, they are almost exactly the
39:32same height.
39:32The south tower is just slightly, slightly taller than the north, only by 4 centimeters, which is really remarkable.
39:40Also, if you consider the age of the two towers, really tells you about the craftsmanship and the skills of
39:47the people that built this building.
39:51Doug's scans are a precise digital copy of the cathedral.
39:56They reveal how generations of craftsmen remain true to a design that's more than 700 years old.
40:05When you compare this building with contemporary buildings, what is truly amazing here is that everything has been made by
40:11hand, by craftsmen.
40:13Everything from individual stones to the entire structure.
40:16And that really is significant.
40:18It isn't manufactured off-site.
40:21It isn't coming out of a factory.
40:23This was done by hand.
40:25The vision behind it by the people that wanted this build is incredible.
40:30It is a really significant building in its size and its beauty and its grandeur.
40:41Cologne Cathedral, like its predecessor at Amiens, is an extraordinary achievement.
40:49Today, millions of people pass through their richly decorated doors to gaze at the first skyscrapers, which took many lifetimes
40:57to complete.
41:05Cologne Cathedral continues to inspire awe and devotion today.
41:11Thanks to its soaring Gothic arches, which revolutionized architecture.
41:18And a dazzling interior, awash with sunlight from thousands of shards of painted glass.
41:33Cologne's sheer size and complexity continue to amaze engineers today.
41:46Its two iconic towers stand out like beacons, drawing visitors from across the world to this medieval wonder.
41:56To theно voll�� blev in that scene of glamour.
41:58ALL VAR
41:58To the karst
41:59To the dark
41:59To the dark
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