The iconic twin towers of downtown Manhattan’s World Trade Center were a triumph of human imagination and will. Completed in 1973, the towers stood at 110 stories each, accommodating 50,000 workers and 200,000 daily visitors in 10 million square feet of space. They were the hub of the bustling Financial District, a top tourist attraction and a symbol of New York City’s–and America’s–steadfast devotion to progress and the future. On September 11, 2001, the World Trade Center became the target of a massive terrorist attack that took the lives of nearly 3,000 people. The disaster also radically altered the skyline of New York City, destroying the twin columns of glass and steel that over the years had come to embody the city itself.

World Trade Center: A Dream Is Born

The 1939 New York World’s Fair included an exhibit called the World Trade Center that was dedicated to the concept of “world peace through trade.” Seven years later, one of the exhibit’s organizers, Winthrop W. Aldrich, headed a new state agency with the proposed goal of creating a permanent trade exposition based in New York. Market research indicated that the city would benefit more by modernizing its ports, however, and the plan was soon scrapped.

Did you know? More than 10,000 workers were involved in building the World Trade Center complex.

Aldrich’s nephew, David Rockefeller, didn’t forget the idea. The grandson of Standard Oil founder John D. Rockefeller, David decided to revive the World Trade Center concept as the core of a revitalized lower Manhattan. In May 1959, Rockefeller formed the Downtown-Lower Manhattan Association, which planned a $250 million complex near the Fulton Fish Market on the East River, including a single 70-story office tower and several smaller buildings.

The Port Authority Signs On

For the resources and power to make the project work, Rockefeller turned to the Port of New York Authority. The Port Authority had been chartered in 1921 by New York and New Jersey to build and operate all transportation terminals and facilities within a 25-mile radius of the Statue of Liberty. By 1960, after constructing the Lincoln Tunnel and the George Washington Bridge, the Port Authority was rapidly expanding its influence, with 5,000 employees and more than $1 billion in freight and transportation structures all presided over by its powerful director, Austin J. Tobin.

The Port Authority had just agreed to take over and renovate New Jersey’s Hudson and Manhattan commuter railroad, the PATH (Port Authority Trans Hudson) train, built in 1908. The PATH terminal was on the west side of Lower Manhattan, and Tobin’s team decided to move the prospective trade center location from east to west, combining the two projects. A region bounded by Vesey, Church, Liberty and West Streets–known as “Radio Row” for its many consumer electronics shops–would have to be razed for the trade center to be built. After a bitter legal battle with representatives of the Radio Row merchants, the Port Authority won the right to continue its plan.

Sights Set on Record-Breaking Height

By this time, the Port Authority had decided that the trade center should replace the 1,250-foot-high Empire State Building, built in 1931, as the world’s tallest building. To fulfill the Port Authority’s requirement, architect Minoru Yamasaki designed two towers of 110 stories each. Instead of the traditional stacked glass-and-steel box construction of many New York skyscrapers, Yamasaki worked with structural engineers to come up with a revolutionary design: two hollow tubes, supported by closely spaced steel columns encased in aluminum. Floor trusses connected this exterior steel lattice to the central steel core of the building. In this way, the “skin” of the building would be strong enough that internal columns wouldn’t be necessary to hold it together.

Construction began in February 1967, after the Port Authority faced down criticism about the towers’ safety and viability from many powerful figures, including real estate tycoon (and Empire State Building owner) Lawrence Wien. Wien even ran an ad in the New York Times in May 1968 predicting that a commercial airliner was likely to fly into the towers. Plans had already been made to guard against such an accident–which had happened in July 1945 with a smaller plane at the Empire State–and the towers were designed to be safe in a collision with a fully loaded 707 plane (the largest existing plane at the time). It was assumed such a plane would have to be lost in fog for such an event to occur; a terrorist attack was never envisioned.

Feats of Engineering at the World Trade Center

View of the World Trade Center under construction, with a sign announcing the completion schedule, circa 1969. Hulton Archive/Getty Images

Because the ground in lower Manhattan was largely landfill, engineers would have to dig down 70 feet to reach bedrock. Excavating machines dug a three-foot-wide trench down to the bedrock, and as dirt and rock were removed, they were replaced by slurry: a mixture of water and bentonite, a type of clay that expands when wet to plug any hole along the side of the trench. Workers then lowered a 22-ton, seven-story-high steel cage into the trench and filled it with concrete by using a long pipe. As the concrete flowed in, it displaced the bentonite slurry.

By making more than 150 of these slurry trench segments, workers enclosed an area two blocks wide and four blocks long. Called the “bathtub,” it was used to seal the basements of the towers and keep water from the Hudson River out of the foundation. All in all, one million cubic yards of landfill had to be removed. The Port Authority used this landfill to create the $90 million worth of land that would become Battery Park City. To piece the steel frame of the building together, engineers brought in Australian-made “kangaroo” cranes, self-powered cranes powered by diesel motors that could hoist themselves up as the building grew higher.

At the end of construction, these cranes had to be disassembled and brought down by elevator. When the towers were finished, each one would have 97 passenger elevators, capable of carrying loads of up to 10,000 pounds at speeds of up to 1,600 feet per minute. In all, the towers were assembled from more than 200,000 pieces of steel manufactured around the country, 3,000 miles of electrical wiring, 425,000 cubic yards of concrete, 40,000 doors, 43,600 windows and six acres of marble.

World Trade Center: A Dream Come True

The last piece of steel was put in place on the north tower (One World Trade Center) on December 23, 1970; the south tower (Two World Trade Center) was topped off in July of the next year. Construction went on until April 1973, when the five-acre outdoor plaza, dominated by a 25-foot-tall bronze sculpture by Fritz Koenig, was completed. At the official ribbon cutting ceremony on April 4, Governor Nelson Rockefeller (David’s brother) proclaimed triumphantly, “It’s not too often that we see a dream come true. Today, we have.”

At 1,360 feet, the World Trade Center towers were the tallest buildings in the world for less than a year; they were soon surpassed by Chicago’s Sears Tower. Still, the towers held an incomparable mystique. They inspired incredible stunts, beginning in August 1974, when Philippe Petit walked a high wire between the two towers.

In May 1977, George Willig earned himself the nickname of “the Human Fly” by hoisting himself to the top of the south tower using homemade climbing devices. The Port Authority loved these stunts because they endeared the towers to the public and made them seem like giant toys. They worked at turning the towers into an attraction, adding the Windows on the World restaurant, which opened on the 107th floor of the north tower in April 1976 and was an immediate hit.

By 1983, World Trade Center revenues had jumped to $204 million, and space was in high demand. Smaller importers-exporters were now being pushed out by rising rents, making way for major businesses.

1993 Bombing of the World Trade Center

New York City Police officers view the damage caused by a truck bomb that exploded in the garage of New York’s World Trade Center, 1993, that killed six people and injured more than 1,000. (Credit: Richard Drew/AP/REX/Shutterstock)

The first major test of the trade center’s structural integrity came on February 26, 1993, when a bomb with the destructive power equal to 2,200 pounds of TNT exploded in the parking garage of the second floor basement of the north tower. The blast killed six people, injured more than 1,000 others and caused an estimated $600 million in damage. Six Islamic extremists were tried and convicted in connection with the plot.

The towers reopened 20 days after the bombing with new security measures in place, including restrictions to parking lot access and electronic identification badges for building tenants. Over the next eight years, the Port Authority spent a total of $700 million on renovations, with safety upgrades like battery-powered stairway lights and a separate emergency command center in each building. Mayor Rudy Giuliani set up a high-tech emergency operations command center, dubbed “the Bunker,” at 7 World Trade Center, a 47-story office building adjoining the towers.

The World Trade Center on September 11th

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In July 2001, just two months before the terrorist attacks of September 11, the Port Authority agreed to lease the twin towers to Larry Silverstein, a New York City developer. Silverstein agreed to pay the equivalent of $3.2 billion over the next 99 years. At the time, over 99 percent of the 10.4 million square feet controlled by the Port Authority was occupied.

The impact of the two planes that hit the World Trade Center towers on September 11, 2001, was more devastating than any of the building’s designers and engineers had ever imagined. The first plane ripped a hole in the north tower from the 94th to the 98th floors, causing massive structural damage and igniting some 3,000 of the 10,000 gallons of jet fuel the plane was carrying. The second plane hit the south tower at an even faster speed, striking the corner and gashing the building from the 84th to the 78th floors.

The heroic efforts of the city’s fire and police departments and other emergency services helped 25,000 people escape from the site on 9/11 before the unthinkable occurred. The damage done at each point of impact forced the physical weight of the towers to be redistributed, and the undamaged part below the hole had to support the floors above. At the same time, the fires raging in both buildings weakened the steel trusses holding up each floor. With damage to a greater number of floors lower down on the building, the south tower gave way first, crumbling to the ground at 9:59 a.m., only 56 minutes after being hit. The north tower collapsed less than a half hour later, at 10:28 a.m.

Debris from the falling towers ignited fires in the remaining buildings of the trade center complex, including 7 World Trade, which burned for most of the day before collapsing at 5:20 p.m. Overwhelmed by horror, shock and grief, New Yorkers and people around the world trained their eyes on “Ground Zero,” where the fall of a treasured icon of American industry and ingenuity had left a gaping hole in the sky.

READ MORE: How the Design of the World Trade Center Claimed Lives on 9/11

One World Trade Center

That hole in the sky would eventually be filled by One World Trade Center, or “The Freedom Tower,” which rises even higher than the Twin Towers it was built to honor. At a symbolic 1,776 feet tall, One World Trade is the tallest building in the United States and Western Hemisphere, overtaking Sears Tower in Chicago. Built on the original 6 World Trade Center, it was originally designed by architect Daniel Libeskind to be an asymmetrical tower inspired by the Statue of Liberty.

In 2004, architect David Childs, known for designing both the Burj Khalifa and the Willis Tower, took over. The cornerstone was laid on July 4, 2004, but the building did not open until November 3, 2014. Architecture critic Kurt Andersen wrote, “The fact that it’s taken more than a decade to finish, I think —the gradualism—makes that sense of emblematic rebirth more acute and irresistible.”

One World Trade is 104 stories tall and has three million square feet of office space topped off by One World Observatory, an observation deck, bar, and restaurant open to the pubic. It spans from floors 100-102 and offers visitors panoramic views of New York City.

Rebuilding the World Trade Center

A new tower at 7 World Trade Center opened in 2006. The $2 billion 4 World Trade Center followed in 2013. The World Trade Center Oculus, a glass and steel transit concourse and shopping center designed by the Spanish architect Santiago Calatrava, opened to the public in 2016, while the 1,155 foot tall 3 World Trade Center opened in 2018. Silverstein’s 2 World Trade Center and 5 World Trade Center remain incomplete.

The rebuilt 16-acre World Trade Center site also includes the National 9/11 Memorial designed by Michael Arad. His design, “Reflecting Absence,” includes two reflecting pools in the footprints of the former Twin Towers surrounded by bronze panels with the names of all 2,983 victims of the 1993 and 2001 World Trade Center attacks.