If you hold your breath and dive down 33 feet, your lungs contract in size by a factor of two because there is twice as much pressure around the air in your lungs, making them contract. When you rise back up, the air expands again and your lungs return to normal size.

When you breathe from a scuba tank however, the air coming out of the tank has to have the same pressure as the pressure that the water is exerting. Therefore, when scuba diving, the air in your lungs at a 33-foot depth has twice the pressure of air on land and the pressure continues to double as the depth gets deeper.

“If a diver stays under a low depth of water for a certain period of time, some amount of nitrogen from the air will dissolve inside them”

Imagine a carbonated drink, Carbonation in liquid is made through high-pressure gases in the air coming in contact with water and dissolving.  When you release the pressure in a bottle of carbonated liquid, the dissolved gas quickly rises as bubbles.

If a diver stays under a low depth of water for a certain period of time when diving, some amount of nitrogen from the air will dissolve inside them, much like the gas in carbonated drinks. If they swim to the surface quickly, the dissolves gasses inside their body will react in much the same way as those inside a carbonated drink and come to surface in bubbles.

This occurrence in a human body is called decompression sickness and can cause many problems, at very worst it can be fatal to a diver.