Help Me

We don't do homework here, sorry.

You can research it by typing into Google or Wikipedia the following terms:

Rapture of the deep
Nitrogen narcosis
Decompression sickness (divers' disease, the bends or caisson disease)

Here's a start:
http://en.wikipedia.org/wiki/Decompression_sickness

http://en.wikipedia.org/wiki/Nitrogen_narcosis

The how is simple, water pressure.

At the surface, there is the weight of one atmosphere on us - 14.7 pounds of force per square inch. Water, a fluid, weighs more than air - each 33 feet of water equals the weight of the atmosphere. The force exerted upon a body is the same whether it's sea water or fresh water in a lake.

At 66 feet a diver has 3 atmospheres of pressure on their body, one from the air and two from the water. A diver doesn't really feel the change in pressure except in the ears, which have to be "cleared"/adjusted to the pressure change as a diver descends or the mask which is compressed against the diver's face. Blowing a small amount of air out thru the nose equalizes that pressure.