Building the Twin Towers

When terrorists flew planes into both Twin Towers on a crystal clear September day in 2001, horror mingled with disbelief and, even in the midst of the chaos, with rage. No one who saw the towers fall will ever forget it. But as time passed, the sight of a skyline without the Twin Towers became more than just a reminder of a savage attack and the loss of thousands of lives. It also brought home to New Yorkers -- and to millions of others around the world -- that they'd somehow grown improbably fond of the two severe, oddly companionable monoliths anchoring Lower Manhattan. Here, LIFE.com pays tribute to those engineering marvels ...