About two years ago I took the SCUBA class at my university. It was a weekly class where we practiced in the pool throughout the semester, and then went on a little road trip at the end to go get our certification at a flooded quarry.

I went through the class without too much trouble, but I did have trouble clearing my mask. When the water in the pool hit my face, I had trouble controlling my breathing well enough to fill my mask. The sensation was hard to describe, but I just had trouble breathing out and into my mask. But with some practice, I mastered this in the pool.

Then we went to sit our test at the quarry. This was my first time being so deep underwater. I think I was about 15 or 20 feet under. It had taken me forever to get there because I had a lot of trouble equalizing that day. Also, I don't deal very well with cold water, and this water was just unbearably cold, so I wasn't very happy that day.

The instructor was floating around between us having each of us complete various tasks to pass our test. When he came to me, he gave me the motion to have me clear my mask. So I pulled off my mask. And the moment the cold water hit my face, I completely lost the ability to breathe at all...in OR out. I don't know how to describe the sensation at all; it was very bizarre. I thought there was something wrong with my regulator, so I switched to my other regulator. Still couldn't breathe. I did the "no air" hand signal and my instructor handed me his regulator. Still couldn't breathe. At this point I had been without air for about 20 seconds and I was starting to panic. I did exactly what you're not supposed to do, and inflated by BCD a little bit and returned to the surface WAY faster than I safely should have. Keep in mind I was COMPLETELY unable to breathe in OR OUT the entire way up. So I was holding air in my lungs. I don't know how I didn't rupture a lung, but somehow I didn't. I think perhaps the random point in my breathing cycle that I was at at the moment when I removed my mask may have saved my life; had I had a full lung of air at that moment, I can't imagine I would have managed not to pop a lung.

Upon returning to the surface and checking all my equipment, and having my instructor re-check it, all of my equipment was functioning fine and I had plenty of air in my tank. The instructor was a bit of an ass about it all. He just gave me a very angry lecture about how I shouldn't have inflated my BCD, and he wouldn't discuss with me the fact that I was totally unable to breathe. I had paid a lot of money to get certified, and could have re-done the exam for free, but I was so freaked out by the experience that I never did.

I've since attributed what happened to some weird human form of the mammalian diving reflex:
http://en.wikipedi...lian_diving_reflex

But I don't really know for sure.