Alan Weisman discusses his book, World Without Us, at the Long Now.
He is mostly surprised at how quickly things would fall. Manhattan, for example. The sewers would totally flood in 36 hours. This would corrode all the supporting structures (which are not designed to be wet), leading the streets above to collapse within, say, 20 years. The skyscrapers wouldn’t last much longer.
Picturefest clipped from various posts. All images linked to source article, though most appear to originate from the above mentioned slideshow.