I've heard that when a diver surfaces quickly, they can expel a single breath all the way up. The air in their lungs continues to expand as they go through the pressure gradient.
Has anyone actually experienced this? I'm assuming it would have to be a really quick surface (ie: dropping your weights), and after only a short dive or on pure O2.
As a relatively experienced scuba diver I can say that the theory is correct. I haven't done an emergency ascent, as you describe, but standard practice is that you exhale on the whole way up or you will burst your lungs.
So by ascending from 30 meters to the surface you expand the volume of the air in your lungs four times. (Of course, you don't want to do this, as you will most likely get decompression sickness, aka the bends.)
Just let me note -- don't hold your breath when there are pressure differences between your lungs and the surroundings!!
You can find that in any scuba diving manual.
(Edit: I do realize the parent poster probably knows this.)