In 1978, Diane Hartley was writing  her undergraduate architecture thesis when she made a shocking discovery. Her paper focused on the Citicorp Center,  a skyscraper in midtown Manhattan. And after weeks poring over the building’s plans, she’d stumbled on a potentially deadly mistake. An oversight that threatened to topple  the 59-story tower into one of New York City’s  most densely populated districts. When it was built two years earlier, Citicorp Center was one of the world's tallest buildings. Its sloped roof was unique in the city skyline, but its more distinctive feature lay at the base. Since the construction site was already occupied by St. Peter's Lutheran Church, the new skyscraper had to be built  on columns supporting it, like stilts. Using stilts on a building’s corners  wasn’t unheard of, but because the church stood at the corner of the block, these stilts had to be placed at the center of each side. While this novel design worried some of the building's backers, chief structural engineer  William LeMessurier took numerous precautions to ensure  the building’s stability. The outside would consist  of v-shaped chevrons, forming a strong exoskeleton  to support the skyscraper. This external structure also made the building much lighter, meaning there’d be less weight  to support overall. This design did leave the building  vulnerable to strong winds. But LeMessurier had another  state-of-the-art solution— a tuned mass damper. This 400-ton counterweight was controlled by computerized sensors designed to counteract any swaying. With these structures in place, calculations showed that each side of the  building could withstand powerful winds. And with all safety issues resolved,  the building opened for business in 1977. But when Hartley was studying the tower  a year later, she noticed something odd. It was true that each face of the building could endure powerful winds. And since a building’s broad sides catch the most wind, these would typically be the strongest winds a building encounters. However, the towers unique base meant that winds blowing on the building’s corners  were actually the bigger threat. And since traditional designs didn't warrant safety calculations for corner winds, it seemed to Hartley that the threat  had gone unaccounted for. When Hartley contacted LeMessurier’s firm about the issue they assured her the building was strong enough to handle these winds. But checking the plans again, LeMessurier noticed an alarming detail. A change approved without his knowledge had replaced the exoskeleton’s  welded joints with cheaper and weaker bolted joints. This alone wasn’t enough to topple  the tower thanks to the mass damper. But if a storm knocked out the building's power, it would deactivate  the counterweight’s sensors, leaving the building vulnerable to winds  of just 112 kilometers per hour. Given available weather data, a storm this strong had  a one-in-sixteen chance of hitting New York City  every single year. LeMessurier never told Hartley  what she’d uncovered. In fact, everything he did next  was top secret. After filling in the architects  and executives at Citicorp, LeMessurier’s team worked with city  officials to craft a confidential plan. Without warning the residents, construction crews began  a string of night-time shifts to reinforce the bolted joints. This delicate work began  in mid-August 1978, and was only halfway complete when Hurricane Ella approached the city in September. City officials and Citicorp executives  planned an emergency evacuation for a 10-block radius, but at the last minute,  the hurricane veered out to sea. These secret evacuation plans  were never used, and the reinforcements were completed  just a month later. Typically, it would’ve been impossible for this covert construction to go unnoticed. But the press was occupied  with a newspaper strike spanning the length  of the reinforcement project. In fact, the public didn't learn how close they'd come to disaster until 1995, when a New Yorker article revealed the story to the city, and to Diane Hartley. Like LeMessurier, the article failed  to give credit where it was due, but at least Hartley knew  that her homework had saved lives.