Its discovery was "completely unexpected."        

Scientists have discovered a massive underwater volcano off of Indonesia's western coast of of Sumatra Island. The 15,000-foot-high mountain spans 30 miles, said government marine scientist Yusuf Surachman Djajadihardja. It is not clear whether the volcano is active, but he said that if it is and it erupts, it will be "very, very dangerous."

Is it any wonder that Indonesia has some of the best diving?  Just remember, every thing comes with it's own set of risks.