**OH! I originally did this with a 2L pop bottle (yes, pop, I'm Canadian) but it was really hard to make him dive, so I swapped it to a 500ml bottle and that fixed the problem
So obviously the 'science' in this is buoyancy and density, if it really boggles your mind you can read more about traditional cartesian divers here: http://en.wikipedia.org/wiki/Cartesian_diver yeah yeah it's a wiki link but it seems to have all the right info.
Go, be with squid, be with bliss
Here is the little guy in action!