here's a question for the physics geeks?

i would like to know why it is that when i take a 2 liter soda bottle , rinse it out and fill it ( leaving about 2 inches air space for expansion)with hot tap water, lay it on it's side in a deep freezer that not only does it have an air bubble where you would normally expect to find one, at the top, but why is it that it also forms an air bubble on the bottom side as well. if you think i'm full of moocaca, then try it out, you'll see what appears to be a law of physics being broken. though i'm sure there is no breakage of physics law being accomplished it is rather strange that an air bubble would form on the bottom. can some one explain this phenomenon?