Cavitation

Cavitation is the phenomenon where small cavities of partial vacuum form in fluid then rapidly collapse, producing a sharp sound.

Cavitation is believed to be the cause of cracking knuckles. Cavitation by the propellers of submarines are the giveaway of the vessels' stealthiness. The phenomenon known as supercavitation is used to allow objects to travel under water at high speed.

External links:

Super cavitation (http://www.cyberdenone.com/digital_diary/digital_diary_12.html)