Two years before the official opening of the Twin Towers in 1973, PopSci reporter Robert Gannon visited the 110-story construction site – and nearly chickened out. To reach the top of the world's tallest building, Gannon had to climb wobbly wooden ladders lashed to metal beams (while the builders teased him for his "hippie mustache.") "More and more open spaces in the building's skeleton, my boggled mind noted. I looked across to New Jersey. The day was one of those rare clear ones when New York City shimmers in the crystal air."