hmmmmm....it's been many, many years since my basic SCUBA course, but if I remember correctly, for every 33 feet you descend, the pressure increases by 1 atmosphere. So, at 40 feet you'd be at a little over 2 atmospheres of pressure, at 60 feet you'd be at almost 3 atmospheres....

Basically, what this means is that if you are sitting in 33 feet of water, take a full lungfull of air from a tank and ascend while holding your breath...then the amount of air in your lungs would double by the time you reach the surface....result would be a blown lung or an embolism....neither of which is desireable :-)