Because it can seriously injure or even kill you.
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Because a rapid ascent from the pressure of the deep can cause nitrogen bubbles to form inside your body. It is like taking the pressure off of a can of soda and watching the gas fizz all over. Now imagine a similar even taking place in your blood and the fluid in your tissues! This causes pain in the joints, and can even block the flow of blood in your body. Chaz
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The body absorbs nitrogen at any depth. Due to the pressure as you go deeper, you absorb the gas quicker than if you were near the surface. These gas bubbles expand as you rise from the depths. A good example is to take a blown up balloon with you when you go swimming. Watch what happens when you pull it deeper underwater. Then release it. Imagine your lungs were full at depth and you rise too quickly! They could not expand as a balloon could and would burst.