Born into a poor family of Japanese immigrants in Seattle, Washington, Minoru Yamasaki put himself through college working in fish canneries in Alaska. He started his career in New York, working for the firm that built the Empire State Building, and rose to helm his own firm in Detroit. By 1962, when Yamasaki applied to design the World Trade Center, he had completed work on a single high-rise building: Detroit’s Michigan Consolidated Gas tower, which had just 30 stories.

The Port Authority chose Yamasaki based on his proposal to design a vast trade center that still had the intimate, human-focused qualities of his other designs. Tasked with building the world’s tallest building, Yamasaki settled on a design of two towers and five other buildings that would together comprise some 15 million square feet of office space.

Perhaps motivated by self-interest as well as concern, a group of leading New York City real estate developers (unsuccessfully) challenged the Port Authority to scale down its proposal for the World Trade Center beginning in 1964. Led by Lawrence Wien, an owner of the Empire State Building, the Committee for a Reasonable World Trade Center joined a growing number of critics arguing that the twin towers would be unstable at such a massive height—and unsafe in the case of an airplane collision or fire. In 1968, Wien even took out a nearly full-page newspaper ad with an artist’s rendition of a commercial airplane about to fly straight into the upper stories of the North Tower.

Constructing what was then the tallest building in the world posed one of the most challenging foundation projects ever faced on the island of Manhattan. The chosen site for the project was built on landfill that had gradually extended the west side of Lower Manhattan into the Hudson some 700 feet over the centuries. Building the foundation of the twin towers required digging 70 feet to the bedrock and excavating more than 1 million cubic yards of dirt.

To avoid flooding the site, workers dug a 3,500-foot-long, three-foot-wide trench around the perimeter of the site (comprised of more than 150 22-foot-long sections) and filled it with a slurry made from water and bentonite, an absorbent type of clay. Because the slurry was denser than the dirt that surrounded it, it prevented the dirt from filling the trench. Steel cages some seven stories high and weighing 25 tons each were then lowered inside the trench panels and concrete poured around it, forcing the lighter slurry up and out. The completed foundation was commonly known as “the bathtub”—though it was a tub that kept water out, and not in.

Traditional skyscrapers owed their stability to a system of large vertical columns running through each floor at intervals of 15-30 feet, with the exterior walls providing little support on their own. But in order to open up the vast swaths of office space called for in the planned twin towers, engineers put the bulk of the buildings’ strength outside, essentially creating stiff tubes of heavy steel. The innovative design allowed for minimal columns inside, most of them clustered at the building’s center so as to maximize the amount of open space on each floor.

The builders of the World Trade Center put extensive research into the effect of wind on the towers, commissioning one of the earliest wind tunnel studies for a skyscraper and performing perceptual tests disguised as eye exams on unsuspecting subjects to figure how much the building could sway in high winds without people noticing. To ultimately mitigate the wind’s effects, engineers distributed more than 10,000 “viscoelastic dampers”—made of a viscous but still flexible combination of metal, epoxy and polyacrylic glue—throughout each tower. With this cutting-edge shock absorbing system in place, the towers were designed to be able to sway up to three feet in either direction on a windy day.

In all, workers used some 200,000 tons of steel to construct the twin towers of the World Trade Center. But getting that steel to the top of the building during construction posed a thorny challenge, since the towers’ planned height topped even the world’s tallest cranes at the time. So engineers turned to an Australian-designed, self-contained crane nicknamed the “kangaroo crane” that used heavy-duty hydraulics to “jump” upward as many as three floors at a time as the towers rose.

With 110 stories in each tower, engineers faced the challenge of designing an elevator system that wouldn’t eat up a gigantic share of the available space inside the building. Their solution mimicked the city’s express and local subway system: They split the building into three zones, each served by an express elevator. People would get off at one of the building’s “sky lobbies,” then take local elevators to get to their desired floor. This system enabled engineers to use up far less space for elevator shafts, greatly increasing the amount of rentable space in each tower.

In addition to the elevators, the decision to place the building’s staircases in the center of each tower was also designed to maximize open office space. At the time the World Trade Center was built, city building codes required six staircases for buildings of the towers’ height. But as an interstate agency, the Port Authority was exempt from such codes. It chose to follow a newer building code that required only three staircases, a decision that would have devastating consequences on 9/11.