The Freedom Tower’s designers had to contemplate the whole horsemen-of-the-apocalypse spectrum of possibilities: explosives big and small; fire; chemical, biological and nuclear attack. But the most obvious goal of the design team—headed by the architectural firm of Skidmore, Owings and Merrill—was to create a structure robust enough to avoid a reprise of the twin towers’ fate: catastrophic failure as the buildings buckled under their own weight, 110 stories pancaking down in 10 to 15 seconds.

The idea of building a tower to
touch the sky goes back thousands of years. And within the past century, architects and engineers have designed seemingly impossible structures that stand a quarter-mile high -- a tribute to humanity's need to test the limits, as well as a way to alleviate congestion in crowded cities. But after terrorists crashed two hijacked passenger planes into New York City's tallest buildings on September 11, leveling both of the Twin Towers, iconic skyscrapers around the world suddenly gained a new label: target.