it is one of the greatest engineering feats of all time a miracle made a brick and mortar that defied the doubters and helped inspire the Renaissance the Dome of the Florence Cathedral built six hundred years ago by a goldsmith with no training as an architect who dared to attempt the impossible he constructs the dome at a time where the technology should not have permitted it it just should not have been possible constructed of over 4 million bricks weighing 40,000 tons it's the largest masonry dome the world has ever known yet the methods used to build it remain a mystery now a professor obsessed with the dome secrets joins forces with a team of American bricklayers to put his theory test I had to figure it out myself because the Builder didn't write anything down and he didn't leave anything behind and this must be the way they have done it this would be the only way that could make sense great Cathedral mystery right now on this Nova National Geographic special [Music] major funding for Nova is provided by the following the David H Koch fund for science supporting Nova and promoting public understanding of science and the Corporation for Public Broadcasting and by PBS viewers like you thank you additional funding from Rodger and Vickie sad [Music] Florence Italy [Music] for centuries travelers have been coming here to gaze at the wonders of the Renaissance [Music] 600 years ago this city experienced a creative explosion unlike any other visionaries like Leonardo da Vinci and Michelangelo flourished here in an atmosphere that celebrated imagination and innovation [Music] many believe if the Renaissance began with the completion of the city's most visible landmark the dome on the cathedral of santa maria del fiore built sixty years before Columbus sailed the Atlantic without the use of modern machines or materials the dome is still the largest of its kind in the world it's an icon shrouded in mystery because no one knows exactly how it was constructed certain features of the domes stand out it's in the shape of a pointed arch with eight sides rising to a central point topped by an enormous marble Lantern but there is more to it than meets the eye [Music] the exterior tiles conceal walls containing over four and million bricks and what appears to be a single solid structure is actually two domes one inside the other the interior dome covers an open space nearly half the length of a football field while the outer shell Rises 10 stories atop cathedral walls themselves a hundred and seventy feet high questions about how the dome was built persists to this day [Music] six centuries ago how could builders work at such great heights how could they know the eight sides would meet in the center and how did the steep brick walls hold together without collapsing with the dome florence moves into an entirely different dimension the dome becomes the hub of a new city of a new world this is so soaring so daring so confident so absolute a structure it's like a work of God [Music] but the dome is the work of a man one of the most elusive and enigmatic geniuses of all time his name was Filippo Brunelleschi trained as a goldsmith he had no experience in architecture or building yet he took on what seemed to be impossible keeping 40,000 tons of masonry curving through the air without caving in to the floor below experts are still trying to understand how he managed to define gravity it constructs the dome at a time where the technology should not have permitted it how is it possible that he built the thing when he did it just should not have been possible by all accounts filippo brunelleschi was a suspicious and secretive man unlike Leonardo he left behind no notebooks no drawings no blueprints for later generations to study so for centuries scholars have been trying to uncover the secret of Brunelleschi's dome you [Music] you that have been spreading across Europe for over a hundred years they consider them too cluttered with their walls propped up by flying buttresses and their many tall pointed spires for inspiration the committee looks to ancient Rome in particular to the classical temple honoring all Roman gods the Pantheon it was famed for its unrivaled dome made of poured concrete but such engineering technology had been completely erased by centuries of war and it's the accepted wisdom of the time that no culture will ever rival the Romans in the building arms Florence is determined to surpass all architectural glories past and present through the 1300s the Cathedral committees vision for santa maria del fiore keeps expanding longer wider higher eventually the committee's reach begins to exceed its grasp they were really presenting themselves with a serious problem because enlarging the church with are also enlarging is the crossing area of the church essentially where the two arms would intersect like many cathedrals santa maria del fiore is in the shape of a cross the larger the church the larger the area over the altar place needed to be covered by the dome they eventually create a cross in space which measured 143 feet six inches across today in the 21st century it would be difficult for us to cover to roof such a vast space in the 14th and 15th centuries theoretically it should have been impossible a mural depicting the Cathedral years before the dome was begun shows what the committee had in mind an enormous pointed dome with eight sides meeting at the top there's no question it's going to be spectacular there's just one catch no one knows how they're going to build it what was so challenging about building a dome on this Cathedral after all a dome is nothing more than an arch rotated 360 degrees and by 1300 Gothic cathedrals had been using arches and vaulting for over a hundred years how did they do it medieval technology relies on wooden frameworks to hold the masonry until the final piece is put in place the two sides pushing against each other allowed the structure to stand on its own this method is known as century in the Middle Ages if we're building a vault if we build that wooden framework we put our blocks our bricks on top of it we wait for the masonry to dry then we make the sign of the Cross pull the wooden framework away and run like hell because the failure rate on most of these vaults was about 50% but this technology would not work in the Florence Cathedral the problem with the wooden centering for santa maria del fiore was that it was going to be unprecedented in scale if they built it it would have been enormous ly expensive the area beneath the dome is so high and so wide just building the wooden framework to support the masonry would have taken hundreds of trees years of construction and huge amounts of money unless someone someday invents a way to keep curving walls in place as they rise the dome will [Music] I mean for me the most extraordinary thing about the construction of the Cathedral is undertaking a project that you knew fully well you did not have the technology to complete [Music] by the time filippo brunelleschi is born the cathedral has already been under construction for 80 years with no solution in sight to the problem of the dome Brunelleschi spends his youth being trained not as an architect or stonemason but in a trade that continues to flourish in Florence to this day precious metals it's a path followed in later years by many artists including Donatella and then Artur da Vinci he began in the workshop when he was 14 years old his father friends workshop he apprenticed until he was 17 or 18 and he learned all the techniques typical of the Florentine tradition to us in the 21st century that may seem a slightly odd way to get your start in architecture but in fact you could have had no better training in the 15th century to become an architect or a sculptor or the designer of any sort they worked with gold they work with silver they used their minds as well as their hands they had to figure out how to make things work both practically and also aesthetically [Music] Brunelleschi first attracts public attention in the year 1401 just 23 he enters a competition to decorate the most revered building in all of Florence the baptistry for centuries Florentines including Dante and the meta G's have been baptized here and the building needs a new set of ornamented bronze doors and filippo brunelleschi being very ambitious and very talented threw his hat into the ring his most important artistic competition for a public work that everyone will see that will immediately create Fame and prestige and he manages to become one of the finalists along with another beginning master Lorenzo Ghiberti the competition involved casting a trial panel making an experimental piece to show what you could do so everyone was given the same amount of bronze and told go away to your workshop and make us something in Travie produce they produce two panels which luckily still survive in these two panels there's a confrontation between the classical style of gibberty and the Renaissance of Brunelleschi a Audrina she meant [Music] lebra mo Darrell Suki Bertie the Abram of Lawrence Liberty is very beautiful she has a long curly beard flowing hair the scene is very decorated it's very rich in detail the one from Brunelleschi is one of incredible humanism echo that's already something you'd look at the way in which Abraham wants to kill his son while the Abram of gibberty is just in his paws with the knife and the Sun is there casually almost like is ready to be stabbed in the won by Brunelleschi is taking his son by the throat and you can see that he has placed his hand where the blood is flowing because you wanted to stand the child because it didn't want the Sun to feel the pain when it stepped in with a knife Krista this is the creation of an incredible genius above all it defines yunalesca as an artist community Kiera that is the difference this brought to the art of 1400 in the Renaissance this is the Renaissance questo ate both hands are masterpieces but Brunelleschi's vision may have been too far ahead of its time the Commission goes to Lorenzo Ghiberti losing the commission to Lorenzo Ghiberti I think is no question hurt Brunelleschi very badly and in many ways shaped his career and the way that he proceeded after that burlesque II may or may not have understood why he lost but certainly from that point Filippo Brunelleschi must feel that he has to carve out a new niche for himself following the competition the disappointed Brunelleschi leaves Florence little is known about his life for the next 15 years but it's clear he spends time in Rome studying the ancient monuments some believe he's already preparing himself for a future challenge building the dome on the Florence Cathedral [Music] here in the nearby park Massimo Ricci's dome is at a critical point he and his helpers are preparing for its biggest test yet with the walls increasing in height Ricci is concerned about having his students continue the work so a new team has arrived in town to help push reaches experiment forward they are all master bricklayers from the United States hi nice to meet you [Music] they're members of the International masonry Institute an organization that trains workers in the craft of brick lane yeah sorry each one has more than 20 years on the job not in Florence for the food or the works of art they are here to lay some brick the bricklayers understand the basic structure Brunelleschi's plan the eight corners of the dome where the walls meet act like the ribs of the dome once these corner ribs meet at the top they form powerful arches together with smaller interior arches this goes a long way toward holding the 40,000 ton mass together basically it's a series of four gothic arches arches that come up this way let me see that this rib here the opposite one there is gonna come up you know like that so you've got a series of four gothic arches it all should meet the middle that's the key working on the model the Americans will be confronting the key mystery of the dome until the curving walls connect at the top what keeps them from falling to the ground and what magic did Brunelleschi used to define gravity you know I've built a lot of things from stadiums baseball football but I've never ever worked on something like this we use different mechanisms to hold arches in places and once we're done we take them out but this is freestanding which I've never same destruction like this their first task will be to literally learned the ropes and begin to understand reaches theory they they are very okay [Music] by 1418 more than 100 years after work had begun the enormous Cathedral is almost complete it's bigger than any other in the world but without a dome it is in danger of becoming the world's largest choke evident is clear the people of the city were worrying about this problem all the Florentines were talking about it they knew very well that they risk looking bad in front of the rivals to latifolia they realize that building has got to the point where they cannot put off any longer how they're going to build this and so they put forth a competition saying that whoever has any ideas about how on earth we can do this we're open it's sort of answers on a postcard please they didn't have any idea what they were going to do no Sofia no kishi proposals for the dome come pouring into the committee but they all share a fatal flaw they depend on using wooden framework to keep the bricks in place during construction only one candidate promises to build a free-standing self-sustaining dome filippo brunelleschi he tells the committee he has figured out a way for the dome to stand on its own even as it is curving inward the financial advantage of that must have been extraordinary but the skepticism was probably even greater in the sense that how could that be possible what will prevent that structure from simply sliding out in caving in as we're building it but there is a problem the 41 year old Brunelleschi has never built anything when they get to that final piece I mean this is really the climax of the entire two century construction history of the church who is this man working in jewelry who now steps forward and says look I have the credentials I have to know how I have the inspiration to actually design the structure and would you trust in him I mean I would not have perhaps Brunelleschi supreme self confidence impresses the committee because he clearly does not have all of the problems worked out in advance [Music] I think that Brunelleschi had a very clear idea of how to build that dome but realize that there were certain construction details that he could only figure out as the work was in progress Filippo being extremely secretive and not wanting anyone else to know his plan said I'll show you how to do it when you give me the job give me the job and I'll begin doing it and you'll see that it works everyone else had shown his plan but unless he refused he said I know how to build it only I know how to build it I've studied the ancient Roman structures I already see it built and and so they said well you have to tell us something so I said bring me an egg and he said anyone who can keep this egg standing upright on the marble tabletop will understand how I'm going to build the dome imagine all of these eminent master masons from all over Europe trying to get it to stand upright on its own all of them fail so they give the egg to burlesque II and say show us what you mean and Azari who tells the story in the 16th century uses a very vulgar term he says people Roop they're cooler war people Filippo's nicknamed him broke the eggs ass so he breaks the bottom of the egg they say well we could have done that too and Brunelleschi says yes and you would be able to build the dome if you know what I know 17 years earlier his radical vision may have cost him the competition for the baptistry doors this time Brunelleschi keeps his ideas secret for as long as possible asking the committee to trust him had he told the assembled company his secret it would have been something that they wouldn't have understood a special brick pattern a special kind of great work that he was going to use in the interior of the dome in April 1420 the committee comes to a decision they choose Brunelleschi along with two others including his old rival gibberty to build the dome if whoever was going to have a moment of doubt I think that would have been the one because he would have seen up close and personal the magnitude of the task that literally lay before him at that point because he would have looked across this chasm this yawning gap it must have had some level gulped and thought am I going to be able to do this Brunelleschi quickly emerges as the leader and takes on his first challenge lifting the building materials 170 feet to the work platform above technologically the means did not exist up until Brunelleschi's time lifting devices were referred to generically as the rota manya okay or is the Great Wheel which was a large wooden wheel that looked very much like a modern gerbil cage inside of which human beings would walk causing the wheel to turn and as that wheel turned it would coil a rope and that coiling would gradually then lift an object based on the lifting power of the people who were actually walking inside Brunelleschi realizes that the old method could not be used in a project this large and a worksite this high he invents a hoist that uses oxen rather than people to raise and lower the loads this is really Bernal ASCII as the engineer barosky is the inventor but they're turning a wheel that would turn a vertical shaft and it in turn would have a series of cogged wheels that would then interlock with other cog wheels and so as the oxen are moving in one direction they could of course lift weight upward okay but more importantly and Brunelleschi realized we're not going to have to only lift weights up we're gonna have to lower those weights as well and so he introduces what is the first ever reverse gear in history keeping the oxen moving in the same direction saves valuable time the hoist raises or lowers material depending on which of two horizontal wheels locks into a vertical wheel on the drum holding the road when a load needs lifting the bottom wheel engages and the drum gathers rope in when a load needs lowering the top wheel is set in place to turn the drum in the opposite direction by simply changing which of the wheels interlocked with the large vertical one you could then change the direction technically or from actually lifting or actually lowering of the material down to the ground the oxen could walk all day long in the same direction keeping materials flowing to and from the work place above in three thousand years of engineering no one had ever done that he pushed beyond a boundary that no one else had crossed no one else had even got to that boundary Brunelleschi crossed over it Brunelleschi had solved the problem of lifting nearly 40,000 tons of material up to the worksite now the former Goldsmith has an even bigger challenge connecting eight massive walls together to form the world's largest dome Florence holds its breath as the walls begin to rise around 1425 five years into the project the bricks by design start to curve inward without a wooden framework to hold the weight the project is entering dangerous uncharted territory the old methods of brick Lange would no longer work [Music] most walls are built by simply laying bricks along straight lines one after another layer upon layer Russell Gentry a professor of engineering at Georgia Tech has studied Brunelleschi's methods so Brunelleschi had built the wall in the simple way you see here you would have layers of brick and layers of mortar and the layers of brick and layer mortar very simply separated by one another and the layers of mortar represent planes of weakness through the wall what we see here is that the wall is leaning in gravity is pulling it in towards me and so a crack could form in one of the layers of mortar the mortar is weaker than the brick and the whole thing could rotate and all of this brick could fall in the time had come for Brunelleschi to share part of his secret plan with the world that's the point in the building where support of some sort was always needed and Brunelleschi had to begin using this special pattern of laying bricks that he himself seems to have invented in Brunelleschi's new design horizontal bricks are interrupted by others set vertically instead of continuing in one straight line the bricks zigzag [Music] in the area between the two domes that pattern is visible today but only in small patches that remain unplastered in Italian the design is called spin a Pesci spine of the fish English speakers call it herring boat [Music] think we're heavens spino ficha for a dinner the herringbone design is even easier to spot in Massimo Ricci's dome the pattern is simple and it's a method the American bricklayers catch on to rapidly they lay the vertical bricks first these are the spines once the spines are set the horizontal breaks are then wedged in between the spines row after row in all their years of working the Americans have never seen bricks laid quite this way it's completely different techniques are definitely definitely different than what we're used to the system is pretty amazing really the vertical bricks in the subpoena Pesce pattern block the mortars planes of weakness this prevents large sections of wall from separating or shearing and tumbling to the ground everything was laid horizontally and you just brought it in slowly each time you would always have a shear point the shear plane where that gets slight off we're here you don't have any single shear point anywhere it's all tied together a million times the herringbone brick pattern is so untested at the time Brunelleschi has had to convince the cathedral board and the workers to allow him to use it once again Brunelleschi is going against convention he's also asking his workers to trust him with their lives they were working around about 220 230 feet in the air and they were literally hanging over an empty space where if they fell it was certain death and so Brunelleschi certainly needed to have his men have faith in him and they needed to believe that Brunelleschi knew what he was doing Brunelleschi must have done something to convince the workers to trust this new method but what the answer may lie in a building that sits just behind the Cathedral in the very shadow of the dome [Music] it was built as a theater in the 1800s many years later it was converted into a parking garage that thought while renovations were going on to build a new wing of the cathedral museum archaeologists dug out centuries of landfill and discovered buried treasure what now appears to be a hole in the ground would have been a free-standing structure in the 15th century the remains of a dome perhaps left there by Brunelleschi himself professor Francesca gulley area of the University of Florence is overseeing the discovery heckuva crystal la famosa cupola here it is this is the famous little dome it was discovered in November 2012 and it surprised the world of architecture el mundo de la cultura are a keystone iike the top has been lost to time but the inward curve of the walls remains the little domes base is constructed of sandstone the brickwork only begins 1/3 of the way up the wall this proportion mirrors exactly the design of the cathedral dome and the brickwork is done in a herringbone pattern here so immediately I was immediately excited about it because having recognized the presence of the herringbone I immediately connected it to Brunelleschi's technique and do you think Brunelleschi's stood here and said this is how I'm going to do it this is the secret to building the dome and multiple yes it's very probable that during the construction Brunelleschi was here to demonstrate the use of the herringbone method la technologie Adela Spina Pesce yes I think this is the model but the new discovery features one obvious difference it's round while the cathedral dome appears to have eight individual walls but appearances can be deceiving [Music] Brunelleschi uses the Spina Pesce pattern to create a dome that looks like an octagon but is actually one continuous spiral [Music] so we're here looking at the corner of Massimo's dome and what we can see behind the corners we see the start of the bedding of the masonry and as we go up we can see the bricks in the spin of pesquet pattern and if you watch carefully you can see that they start here and they wrap up around behind the corner and continue uninterrupted from one face of the dome to the other face of the dome by simply turning some bricks on their sides Brunelleschi establishes a new pattern and achieves two important goals preventing cracks from spreading and binding the eight walls into one unified mass the spiral form resists the forces of gravity more effectively than an eight sided dome for many Spina Pesce is the secret of the dome but the construction of the dome involved many secrets something the American bricklayers have come to understand from their time with Massimo Ricci peace after two weeks on the job they're ready to compare their work to the real thing the interior dome is completely covered by a religious mural Wow the area between the shelves offers only glimpses of key elements and plaster conceals all but a few patches of brick early you can still see here's a good illustration it's fine efficient oh man this is we can't be complaining about anything over our little one-fifth scale on the way down the workers notice something that escapes all but the most experienced I see this guy's see how this is coming up to the center just both of these as that's the that's the horizontal art right there the bricks are sloping down from the corner rib store the Centers of the walls the massive scale of the dome makes it difficult to see but the Americans recognize its significance from Massimo Ricci the important feature barely visible in Brunelleschi's dome becomes clear and reaches model the tops of the walls don't follow a straight line as they go up they dip from corner to corner within each wall this creates an inverted arch one of the most stable forms in architecture these arches combined with the Spina Pesce pattern keep the bricks firmly in place directing the weight of the masonry downward through the walls preventing them from collapsing inward [Music] it's an ingenious design and Ricci struggled for years to figure out how Brunelleschi made it work and how he insured that the walls all met perfectly at the top nearly 300 feet above the ground [Music] Ricci was sure the answer lay in one of the primary building technologies of the time rope lines which Renaissance builders used to guide their work if Ricci could figure out how Brunelleschi set up his ropes he could unveil one of the domes most overlooked secrets he knew the guide ropes must have been attached to a large platform built into the base of the dome but in what pattern I had to figure it out myself because Brunelleschi didn't write anything down and he didn't leave anything behind and Ricci sketched out hundreds of possible designs trying to unlock Brunelleschi secret [Music] then he heard about a 600 year old document stored in the Florentine State Archives it's a drawing done on parchment about five years after work on the dome began it's the only surviving eyewitness sketch of the domes construction and it's a scathing criticism of Brunelleschi's methods the critic named Giovanni de pronto detailed what he considered to be Brunelleschi's mistakes the dome he warned him was doomed to collapse is really tearing him apart he's trying to tear apart Brunelleschi but in effect Giovanni de prata wasn't able to understand much of what Brunelleschi was doing Giovanni de grado de Bravo no idea she's a career I feel informed o que cosa Brunelleschi style fashion the proto may not have understood Brunelleschi system but he was thorough in his observations he carefully sketched the rope lines Brunelleschi was using to guide the construction more importantly for Ricci di Prato also drew the work platform to which the ropes were attached here Ricci noticed something unusual a thin line curving around the platform to Ricci it looked like a flower and he immediately saw it as the key to Brunelleschi system when I saw the platform with the flower drawn on it I said well I'm at the right place Ricci incorporated the flower into the work platform of his model using it to guide each walls construction rope lines are important building tools even today and the Americans catch on quickly to reach a system one worker is stationed on the platform hooks a line to the flower another on the top of the wall handles the other end of the Rope the rope controls the angle and the height of the bricks can you see now hey can you hold it there for a second concentrate on this one regular that's moving the one that bob has a hold of it sets the line up for that spin efficient so it's essentially their guide and that's why the shape of the flower is so important as the platform worker moves the line along the flower the rope transfers the curve of the flower to the wall this creates the inverted arch the walls may be strong but for the dome to work they must meet together at the top one small miscalculation repeated hundreds of thousands of times would lead to disaster as you go up they should follow that all the way up say if this was not correct as you get that dome closing up to the top it's not gonna want to hit in the middle so you might you know the dome might be this way might be this way how could Brunelleschi know his dome was on the right track the answer is at the lines Criss crossing from wall to wall they established the center point and before any rope line guides a brick into place it must pass through that Center brick by brick the walls of Brunelleschi's dome rise until they meet at the top nearly 300 feet in the sky and the dome is complete [Music] for Ricci the ropes guiding the bricks provide the real key to the mystery of the dome a queen I can say with the utmost certainty this is the true secret of santa maria del fiore s in vero segreto de santa maria del fiore after two weeks of working on riches don't the Americans seem convinced by his experiment this must be the way they have done it but without putting a support underneath their this would be the only way that it makes sense this is the flower and it's the herringbone pattern those are the two things that solve the puzzle you wouldn't be able to build a dome like that without those control measures Massimo Ricci has now been working on his dome longer than it took Brunelleschi to build the original his model will remain an open laboratory for those studying Brunelleschi's methods and as an argument for the importance of the flower it's possible that reaching unlike his hero will never see his work finished but if you look at the dates of the building 14:22 14 36 16 years that's the blink of an eye that's very very quick and it's wonderful to think that he saw it completed he was able to look at it he was able to walk past the building and think to himself I will that I did that [Music] with the dome Florence moves into an entirely different dimension the dome becomes the hub of a new city of a new world it is the expression of a self-confidence that no longer knows limits [Music] people need works that can speak to them of their own capacity to dream Brunelleschi's dome is perhaps the biggest of those works in the history of world art [Music] [Music] this nova program is available on DVD to order visit shoppbs.org were called 1-800 play PBS Nova is also available for download on iTunes [Music] [Applause] [Music]