Tom Canavan, 42, should have been long gone by the time the World Trade Center towers collapsed. He worked on the 47th floor of the north tower, processing securities in the trust department of First Union Bank. After American Airlines Flight 11 struck his tower at 8:46 a.m., he delayed leaving to help put securities back in the vault, and then got stuck in a congested stairwell. That's where he was at 9:03 a.m., when the south tower was hit by United Airlines Flight 175. More than an hour after the first crash, Canavan walked through the lobby's revolving doors into an underground shopping mall that connected the north and south towers. This was the primary evacuation route, where rescue workers directed thousands of survivors. He turned around to help an older couple coming through the revolving door, just as a thunderous rumble began. The south tower was falling. Canavan was knocked face-first to the ground and engulfed by smoke and debris. Canavan and another man had to crawl together in nearly total darkness to escape the collapsing tower. They slithered like snakes, heading up and forward, when possible, through a maze of debris. They went over beams and under beams. They pushed aside debris. After 20 minutes, Canavan smelled fresh air. He took a deep breath of cool air. He was on the outdoor plaza near a globe-shaped sculpture that was a World Trade Center landmark. Canavan and the other man had crawled 40 feet east and up through 30 feet of debris, from an underground shopping mall to an outdoor plaza.