The Ending
I am really surprised so many people think he drowned. I thought it was pretty clearly shown that he was in the dark for a while after the cop made the Cosco end collapse, then he found the other end where the sunlight was shining and took the tank off to swim up to the surface.
Plus, it's just not that kind of dark movie.
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See a list of my favourite films here: http://www.flickchart.com/slackerinc