hello, i m dr. sanjay gupta. few of us can forget what unfolded behind me ten years the shocking spectacle of the world trade center towers crumbling into dust against that new york city skyline and the what disappeared along with it. ten years later, the obvious signs of the dust are gone. yet in some waysit still looms over new york city. it still haunts the over new york city. thousands of responders who became sick after breathing it in. in washington, there is a large fire. move it! get out! the world trade center. back, back. what s going on? get out of here! let s go. and the president who is in florida today, within moments after the first tower disintegrated, an eerie silence fell over lower manhattan. there should be a sound when one of the largest skyscrapers in the world collapses. and we know that there must have been a sound. very few people, including myself, remember a sound. dr. david prizant, a pulmonologist and top doctor with the new