This is what it is, according to Wikipedia:
The Bloop is the name given to an ultra-low frequency underwater sound detected by the U.S. National Oceanic and Atmospheric Administration several times during the summer of 1997. The source of the sound remains unknown.
Here is the entire article: http://en.wikipedia.org/wiki/Bloop
Some people say it's a gigantic underwater monster.
I don't know what to believe.
