On a December morning in 1970, the North Tower of the World Trade Center reached the end of its steel. A kangaroo crane swung the final piece into place above the 110th floor, and the men who had ridden that steel up out of a hole in a riverbed watched the last load fly off the machine. Four decades of American skylines ended that day, and the Empire State Building lost its crown to a pair of towers that were still rising side by side. But before any of it could happen—before a single column stood above Lower Manhattan—the builders had to solve a problem no American crane had ever been asked to face, and they had to find the machines that could do it.

In 1968, eight tower cranes arrived in New York from the other side of the world. They were built in Australia, designed to do something no American crane had ever done: sit inside the two tallest buildings ever attempted and climb them as the steel rose beneath. America had been building skyscrapers for seventy years. The Empire State Building, the Chrysler Building, Rockefeller Center—all of them stood as proof of that tradition.

But when engineers drew up the World Trade Center, they ran into a wall. The towers would rise beyond the reach of every crane in the country. So the Port Authority sent for the kangaroos. Everyone remembers how the Twin Towers fell on September 11th, 2001.

This is not that story. This is the story of the seven years when they rose, told the way the machines saw it. First came the fight over sixteen acres of Manhattan and the riverbed problem hiding underneath it. Then the underground wall borrowed from Italy.

The steel war that dragged in mills from Seattle to Japan. And the eight climbing cranes at the heart of it all. It ends at the top, 220 floors of steel later, with the strange puzzle of how you get a crane down from the roof of the world. Before a single machine could touch the site, somebody had to clear it.

And the land the Port Authority wanted was not empty. In the early 1960s, the blocks along Cortlandt Street, Dey Street, and Greenwich Street held a neighborhood New Yorkers called Radio Row. It was the largest concentration of electronics dealers in the world. Surplus radios, vacuum tubes, hi-fi parts, war surplus radar sets—all of it stacked floor to ceiling in narrow shops run by men who had spent their whole lives there.

The city counted 323 commercial tenants in the condemnation zone, along with over a thousand offices and around a hundred residents. The merchants themselves claimed the district supported as many as 30,000 jobs and $300 million a year in business. The push came from the very top of American money. David Rockefeller, the banker spending a fortune to revive Lower Manhattan, wanted a World Trade Complex to anchor the district, and his brother Nelson happened to be governor of New York.

The Port Authority—run for decades by executive director Austin Tobin, with a hard-driving planner named Guy Tozzoli heading the project—had the bonds, the engineers, and the power of eminent domain to make it real. The Port Authority offered each business a flat $3,000 to relocate. It did not matter whether a shop had been there three years or thirty, whether it cleared $10,000 a year or a million. $3,000, take it and go.

The shop owners did not go quietly. A dealer named Oscar Nadel organized the resistance, and the merchants of Radio Row staged a mock funeral in the street. They carried a coffin holding an effigy labeled “Mr. Small Businessman, dead at the hands of eminent domain.

” A sandwich shop in the district, the Courtesy Sandwich Shop, took the Port Authority all the way to the United States Supreme Court, arguing the government had no right to seize private property and hand it to another business. In 1966, the Supreme Court declined to hear the case. The fight was over. Demolition crews moved in that March.

Over the following year, wrecking crews flattened 164 buildings. Whole city blocks disappeared, and the streets between them were erased from the map to create a single sixteen-acre super block. Now the Port Authority owned the land. And that is when the engineers delivered the bad news.

The land was not really land. Most of the site sat on fill that had been dumped into the Hudson River over three centuries. Old cribbing, sunken debris, river muck, and loose rubble ran down roughly seventy feet before anything solid appeared. The bedrock—hard Manhattan schist—lay buried under all of it.

And the groundwater table sat just a few feet below the street. Dig a conventional foundation pit in that ground, and the Hudson would pour in faster than any pump could pull it out. Drive piles, and you would still have water flooding six basement levels, a train station, and the footings of the tallest buildings on Earth. Standard New York foundation work could not solve this.

The answer came from a subway project 4,000 miles away in Milan, Italy. It required digging a trench seventy feet deep through soil that wanted to collapse, while keeping that trench full of liquid the entire time. The method was called the slurry trench, and an Italian contractor named ICOS had patented it in the late 1940s to build subway walls through the waterlogged ground under Milan. When the World Trade Center broke ground in August of 1966, this technique had been used in the United States only twice before.

The Trade Center would be its third American job, and by far its largest anywhere. Here is how the machines pulled it off. A crane held a mechanical clamshell bucket—a steel jaw weighing several tons—and dropped it into the earth along a marked line. Bite by bite, the clamshell chewed a trench three feet wide and about twenty-two feet long, straight down toward bedrock seventy feet below.

The moment each bite came out, crews pumped the hole full of a strange gray liquid called bentonite slurry. Bentonite is an absorbent clay that swells in water. Mixed right, it forms a fluid heavier than the groundwater around it, and that weight presses outward against the trench walls. The soil cannot cave in, and the river cannot push through.

The trench stays open, held by nothing but engineered mud. When a panel reached bedrock, a chisel rig keyed a slot into the rock itself. Then a crane lowered in a reinforcing cage of steel bars—a skeleton several stories tall—down through the slurry. Concrete followed through a steel pipe called a tremie, filling the trench from the bottom up.

Concrete is heavier than bentonite, so as it rose it displaced the slurry, which was pumped off the top and recycled into the next panel. When the concrete cured, the crews moved twenty-two feet down the line, and the clamshell took its next bite. Panel by panel, over more than a year, the wall grew until it ran 3,500 feet around eleven acres of the site. Three feet thick, seventy feet deep, keyed into bedrock, and watertight.

The workers gave it a nickname that stuck for the next sixty years. They called it the bathtub. A bathtub built in reverse, designed to keep the water out instead of in. But a wall like that has a weakness.

Excavate the inside, and the Hudson pushes against the outside with nothing left to brace it. The wall would lean inward and fail. So as the dirt came out, drilling rigs punched diagonal holes through the concrete, down thirty-five feet into the bedrock below. Into each hole went a tendon of high-strength steel, grouted into the rock and tensioned against the wall.

Each anchor was rated to hold between 100 and 300 tons. The crews installed about 1,500 of them, row after row, stitching the bathtub to the bedrock of Manhattan itself. Inside the bathtub, the excavators went to work, and they had company down there. Running straight through the middle of the site were two cast-iron tubes carrying the Hudson and Manhattan Railroad, the commuter line known today as the PATH train.

The trains could not stop. Tens of thousands of New Jersey commuters rode through those tubes every single day. So the excavation simply dug around them. As the soil came away, the century-old tubes were left standing exposed on caissons, like two iron pipelines crossing a canyon with trains still rumbling through while steam shovels worked beneath them.

Crews even cut expansion joints into the exposed iron to handle the changing loads. Those tubes had been carrying commuters since the age of the steam ferry feeding the old Hudson terminal that stood on the site, and they would keep running right through the construction years until a brand new station opened inside the Trade Center itself in 1971. One day a commuter looked out the window of a moving train and saw daylight through a gap in the tube wall. Word spread through the cars.

Somebody pulled the emergency cord, and a trainload of very confused passengers learned they had been riding through an active excavation for months. With the wall holding and the anchors biting into bedrock, the main dig could finally run at full speed. Power shovels and loaders chewed down through three centuries of fill in great descending benches, feeding a parade of dump trucks that ground their way up a haul ramp and out into Manhattan traffic, load after load, month after month. Seventy feet down, the buckets finally rang against the gray schist floor of the island itself—the rock the towers would stand on.

In the end, the machines hauled about 1. 2 million cubic yards of earth and rock out of the bathtub, and almost none of it left the neighborhood. The Port Authority dumped the spoil into cofferdams along the Hudson shoreline next door, creating about twenty-three acres of brand new Manhattan real estate. That landfill became Battery Park City, an entire neighborhood standing today on the dirt that came out of the Trade Center’s basement.

The hole was ready. Now it needed 200,000 tons of steel. And the steel almost did not come, because the biggest steel companies in America tried to corner the job, and the Port Authority decided to go to war instead. In 1966 there were two giants in American structural steel: United States Steel and Bethlehem Steel.

Between them, they had supplied the skeletons of almost every great tower in the country. When the Port Authority asked for bids on the Trade Center steel, everyone assumed one of the two would take the prize. Then the bids came in. Both companies had been circling a number around $80 million.

The offers that landed on the table were both up near $120 million, within a whisker of each other. To the Port Authority, that looked less like competition and more like coordination, and the suspicion was strong enough that the matter drew an antitrust investigation. The Port Authority refused to pay it. Instead, its engineers did something no one had tried on a job this size.

They tore the steel package apart and split it into more than a dozen separate contracts spread across smaller fabricators all over the country and beyond. In January of 1967, the agency awarded about $74 million in steel contracts in a single round—and not one dollar of it went to US Steel or Bethlehem Steel. Thirteen different fabricators would feed the towers. The floor trusses came from Laclede Steel in St.

Louis. The massive welded box columns for the cores were fabricated with plate that came almost entirely from overseas mills, primarily Yawata Iron and Steel of Japan, with more from British rolling mills. The biggest single contract—the prefabricated exterior wall panels from the ninth floor up—went to Pacific Car and Foundry in Seattle, the company known today as PACCAR, the builder of Kenworth and Peterbilt trucks. Their plant turned out roughly 5,700 wall panels, and every one of them crossed the country by rail.

More than 1,600 rail cars rolled east from Washington state to staging yards in New Jersey. From there, the panels moved by barge and truck, many of them rolling through the Holland Tunnel in the small hours of the morning when the streets were empty. Those panels deserve a close look, because they are the reason the towers could rise so fast. Each one was a welded module about ten feet wide and thirty-six feet tall—three stories in a single piece—weighing up to twenty-two tons.

Three hollow box columns, fourteen inches square, joined by three deep spandrel plates. Stand them side by side and bolt them together, and you have an exterior wall that is also the structure itself. Fifty-nine columns marched across every face of the tower, set just forty inches apart. The joints were staggered so that at any given floor, two out of every three columns ran continuous with no splice at all.

Those forty-inch spacings were not an engineering accident. They were the architecture itself. The Port Authority had handed the design to Minoru Yamasaki, a soft-spoken architect from Seattle who had never designed a skyscraper before, and who carried a personal secret that shaped every face of these towers. Yamasaki was afraid of heights.

Standing at a floor-to-ceiling window high above a city made him deeply uneasy. He decided that the people working a quarter mile up deserved to feel held, not exposed. So he pulled the columns close together and made the windows narrow, barely wider than the span of his own shoulders. A person could stand at the glass on the 100th floor, rest a hand on the steel at either side, and feel the building gripping back.

236 of those columns wrapped each tower, 59 to a face, and the structural engineers—John Skilling and Leslie Robertson—turned that architectural choice into the structure itself. Their design made the exterior wall carry the wind. The entire perimeter worked as one enormous hollow tube, a square steel mast a quarter mile tall, braced against every gust off the harbor, while an inner core of 47 heavy columns simply carried the weight of the floors straight down to bedrock. Engineers call the idea the framed tube, and nobody had ever attempted it at anything close to this scale.

And inside that steel hid a detail most documentaries get backwards. The walls of those box columns were not one thickness. Down at the base, where the full weight of 110 stories pressed down, the plate ran as much as four inches thick. Climb the tower, and the plate thinned story by story, until near the top it measured just a quarter of an inch.

The engineers tuned the metal itself, using twelve different grades of steel across the panels with yield strengths running from 42,000 to 100,000 pounds per square inch. Heavy strength where the loads were crushing, light and economical where the wind ruled. A column on the 40th floor and a column on the 100th might look identical from the street. Yet inside the plate they were different animals, matched precisely to the forces each would carry for the life of the building.

You will sometimes hear that the towers were built from steel thin as a coin. That is true only at the very top. At street level, those columns were armor. So the steel was coming, thousands of panels stacking up in the New Jersey yards.

And now the Port Authority hit the problem that defines this story. The Empire State Building had been raised by derricks—simple guide booms that crews leapfrogged up the frame. Slow and crude, but proven. The catch was that every time a derrick moved up, it went out of service.

Days of rigging work for every single jump. And the Trade Center schedule demanded steel rising every working day. There was no room for slow. The towers were designed to outgrow every mobile crane on Earth, and a conventional tower crane planted beside the building could never reach a roof a quarter mile in the sky.

The machine the job required did not exist in America. It existed in Australia. In New South Wales, a company called Favelle Mort Limited had been building a different breed of tower crane: a machine with a hinged luffing jib and a diesel hydraulic power plant that needed no outside electricity. Their cranes were designed to sit inside a rising building and climb it from within.

Because each machine carried its own diesel power plant, it needed no electrical feed from the ground, no cables trailing down a quarter mile of tower—nothing but fuel. Wherever the steel went, the crane could follow. For the Trade Center, Favelle Mort scaled their design up into something the company had never attempted: a model designated STD 2700, roughly twice the size of the biggest crane they had ever built. It could pick up 45 tons close-in, and even with its jib stretched out 150 feet, it could still lift more than fourteen tons.

The steel erector running the job, Karl Koch Erecting of New Jersey, did the math and made the call. With these machines, the towers could be raised with eight cranes instead of twelve slow-working derricks. Eight cranes for two towers, four machines to a building, one standing in each corner of the core—the strongest part of the structure. They arrived in 1968, and New York had never seen anything like them.

The ironworkers watched these strange Australian rigs hop up the buildings week after week, and gave them the name that stuck: kangaroo cranes. The climbing trick worked like this. Each crane stood on a steel mast socketed down into the core, collared to the completed framework. The crane would erect the steel around and above itself, floor after floor, as high as its position allowed.

Then came jumping day. Hydraulic rams beneath the machine extended and pushed the entire crane—cab, jib, counterweight, and all—straight up the mast to the level of the new steel. Koehring guided a fresh mast section into the gap below, pinned it, locked the collars to the frame, and the crane settled onto its new perch, ready to build the next stretch of tower. From the street, the whole operation was nearly invisible.

Just a crane standing a little higher against the clouds than it had the day before. From the cab, it was another world. The operators worked alone in a glass box rising higher than anything around them, reading load charts and wind gusts, swinging 22-ton wall panels onto bolt holes they could barely see. The building raised the crane, and the crane raised the building, step by step, two partners climbing a quarter mile into the sky together.

It would make a better legend to say the kangaroos performed flawlessly. They did not. Favelle Mort had shipped the cranes with a brand-new prototype hydraulic pump design that had never been proven on a job, and certainly not on the frames of the biggest buildings on Earth. Those pumps began to fail.

Koehring crews had to swap them out for older, slower, reliable units, and give back some lifting speed to keep the job moving. Then came the New York winter, and the engineers discovered that machines designed under the Australian sun had a blind spot. The radiators meant to keep the hydraulic oil warm were sized for the Australian climate, not for a Manhattan January hundreds of feet up in the wind. The oil thickened in the cold, and the cranes turned sluggish until the heating problem was solved.

Every machine on a job site has a learning curve. These two towers were the steepest classroom on the planet. The supply line had its own bad days. In 1970, a tugboat strike froze the barges that fed the site.

With steel piling up in New Jersey and the schedule bleeding, the Port Authority tried something desperate: they hired a heavy-lift helicopter to fly the panels across the water. On an early attempt, the load got away, and a multi-ton steel wall panel dropped out of the sky into the harbor. The airlift experiment ended. The strike eventually broke, and the steel went back to moving the slow way.

Through all of it, the kangaroos kept jumping. The first exterior columns went up on the North Tower in August of 1968, with the South Tower starting its steel about five months behind. For years, the two giants climbed the sky in staggered formation, one always a dozen floors ahead of its twin. By late 1969, the North Tower was visibly devouring its own skyline, panels swinging up the faces at a pace no derrick crew could have dreamed of.

And the men bolting that steel together were about to push the building into a race that would put tenants inside a half-finished tower. Here is where the Trade Center build broke every rule in the book. On December 15th, 1970, the first tenants moved into the North Tower and started working at their desks. The tower above their heads was not finished.

Kangaroo cranes were still swinging steel hundreds of feet over the lobby, and office workers rode elevators up into a building that ironworkers were still riveting together. The Port Authority needed rent flowing; the schedule demanded it. And so, for more than two years, the North Tower lived a double life: office building below, active construction site above. At the peak of the work, about 3,500 men were on that site every day.

Ironworker gangs raised and connected the panels. Among them worked Mohawk ironworkers from the Kahnawake reserve near Montreal, part of a tradition of native high-steel men that ran back through generations of New York skyscrapers. Feeding that army was its own logistics problem. Sending thousands of men down a quarter mile of elevators for lunch would have strangled the schedule.

So the meals went up instead, delivered straight to the working floors so the gangs never had to leave the steel. And the steel they were standing on was full of inventions. Years before groundbreaking, in 1964, the design team had put scale models of the towers into a wind tunnel at Colorado State University—the first time in history a building had been through a full boundary layer wind tunnel program, mapping exactly how Manhattan’s gusts would push and twist the finished towers. The engineers even needed to know how much swaying a human being could stand.

So they ran one of the strangest experiments in construction history: they advertised free eye exams to the public. The test rooms were secretly built on hidden machinery. While unsuspecting patients read the eye charts, the entire room slid back and forth around them, and researchers noted exactly when people started to feel uneasy. The answer shaped the structure itself, because the towers were designed to move.

In a strong wind, the top of each tower could sway about three feet from center. To keep that motion below the threshold those eye exam volunteers had revealed, the engineers hid a shock absorber system inside the frame—the first ever installed in a skyscraper anywhere in the world. Small viscoelastic dampers built around a rubbery polymer developed with the 3M company were tucked between the floor trusses and the columns, more than 10,000 of them in each tower. Chief structural engineer Leslie Robertson put the count at about 11,000 per building.

Every one of them quietly turned wind energy into a trace of heat, so that the people inside felt stillness while the building breathed around them. At the very top, between the 107th floor and the roof, the builders tied everything together with a web of steel outriggers the engineers called the hat truss, lacing the inner core to the outer walls so the whole tower finished its climb as one single connected machine. Even the elevators forced an invention. A conventional layout would have eaten the towers alive, because 110 stories of elevator shafts leaves almost no floor left to rent.

Otis Elevator took the contract—worth $35,080,000 for 208 elevators and 49 escalators—and solved the problem with an idea borrowed from the subway: express and local. Giant express cars hauled passengers non-stop to sky lobbies on the 44th and 78th floors, where they transferred to banks of local elevators serving the floors above. Locals in different zones stacked into the same shafts. That single idea raised the usable space on every floor from 62% to 75%.

The big express cars ran at 1,600 feet per minute, among the fastest passenger elevators on Earth. Only one other skyscraper anywhere—the John Hancock Center in Chicago—had ever tried the sky lobby idea before. It is the invention that made the whole project financially possible. And because high-speed cars push a wall of air ahead of them, conventional brick elevator shafts would not survive.

So the team built the shafts from a new gypsum board system instead, making these the first super tall towers ever raised without a single block of masonry. That brings the story to its final stretch. Two beats remain: the day the steel ran out of sky, and a question almost nobody ever thinks to ask afterward. The North Tower topped out on December 23rd, 1970, when a kangaroo crane swung the highest piece of steel into place above the 110th floor.

The South Tower followed on July 19th, 1971. For the first time in four decades, the Empire State Building was no longer the tallest building on Earth. The crown had passed twice, to two towers standing side by side. Topping out is the oldest ritual in steel work—the moment the highest piece goes home and every trade on the site stops to watch it fly.

On those two mornings, one in December cold and one in July heat, the men who had ridden this steel up out of a hole in a riverbed watched the final loads swing off the kangaroos, and Lower Manhattan owned a new roofline. That left the last engineering puzzle. The job was done. Four kangaroo cranes stood on the roof of each tower, a quarter mile in the air.

How do you get them down? You cannot drive them down. No other crane on Earth can reach up to lift them off. The answer was that the kangaroos would take each other apart.

On each roof, one crane was kept in service while crews broke down its three siblings piece by piece. The working crane lowered those pieces down the face of the tower, load after load, a quarter mile to the street. And when only the last crane remained, riggers dismantled it into components small enough for a lighter derrick, which lowered the final pieces and was itself broken down small enough to ride the elevators. The machines that built the towers left the same way the steel had arrived—one careful lift at a time—until the rooftops stood empty.

On April 4th, 1973, the Port Authority cut the ribbon on the finished World Trade Center. The final cost had climbed from an early estimate near $400 million to roughly $900 million. The towers stood 110 stories each, wrapped in slender aluminum-clad columns, holding about 200,000 tons of steel, weighed always by eight Australian cranes. It cost lives to build them, and the record deserves to be stated carefully.

According to the standard published history of the towers—Angus Gillespie’s book Twin Towers—eight men died building the World Trade Center. You will often see a figure of sixty deaths repeated online, but that number traces back to a single safety blog infographic from 2015 with no primary source behind it, and the actual histories do not support it. Eight families paid the price of this skyline, and their loss is part of the building’s true story. There is one more piece of Trade Center history that belongs to the construction itself.

In August of 1974, a French wire walker named Philippe Petit stepped off the roof edge of the South Tower onto a steel cable his crew had smuggled upstairs and walked between the twin towers a quarter mile above the street. The stunt took years of planning, and Petit pulled it off by exploiting the one thing the towers still were: a construction site. He forged identification, posed as a contractor, and made hundreds of scouting trips through floors that were still being fitted out, because in a building full of workmen, two more men with heavy equipment drew no attention at all. Eight times he crossed between the rooftops while police waited on both sides.

The towers were finished, and a man on a wire had given them their housewarming. The kangaroo cranes moved on, and Favelle Favco—the company that built them—became the name in super-scale climbing cranes on towers around the world, a legacy that started on those two rooftops. The bathtub they all stood in is still down there today, holding back the Hudson more than half a century after the clamshells dug it—the oldest surviving worker on the site. The towers themselves stood for twenty-eight years.

And this is how they began. Not with tragedy, but with mud and steel, and eight machines from the far side of the world that climbed a quarter mile of sky, then quietly took each other apart.