Re: Why does air compress underwater?

Just like anything else, air will compress because things under water are 
under higher pressure.

Imagine if you lie down and somebody puts a big stack of books on your 
chest -- you will feel pressure from the weight of all the books.  It is 
the same under water; the pressure is basically from the weight of all the 
water above.  You would compress too if you went under water, just not as 
much as the air because air is much more compressible than your body.  
Under 10 meters of water, the pressure is about twice what it is at the 
surface (where the pressure is basically the weight of the atmosphere 
above).  In the deepest parts of the ocean, it takes specially constructed 
vessels to go down there because the pressure is so large.

Here is a web page that has more information about pressure in the ocean:
 http://newport.pmel.noaa.gov/nemo_cruise98/education/pressure.html