One of my sisters used to worry about the Lincoln Tunnel springing a leak. If that happened, I believe it would be a mass implosion. Bet it would so fast, you'd never know what hit you. ;-)
Manhattan's base is granite... the subways were more of a mining expedition. It is my belief that is why that little island can support all that weight. I don't know if the Hudson river bed is the same but regardless... I believe those tunnels are well engineered.
Moot point for me as I don't expect to be doing any traveling in there.
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