under the sea

Near Fiji, Tonga, and Samoa - with a peak that is about 4000 feet UNDER the ocean - there is the West Mata volcano. The existence of this volcano was discovered in 2009 and an expedition crew filmed its eruption that may.

http://www.noaanews.noaa.gov/stories2009/20091217_volcano2.html

The eruption is caused when magnetic gas expands and pushes volcanic bubbles up into the ocean water. Those lava bubble then come into contact with the significantly cooler water and burst - transforming the scene into what looks like a pressure-cooker exploding with steam.

This was the first filming of what is actually a very common event. Ocean floors are created by these volcanic activities all over the world. The molten lava actually flows for a bit on the seafloor around the submarine volcano.

The commenter notes in the following video that 80%(!) of the eruptions on earth actually happen underneath the ocean. 

https://www.youtube.com/watch?v=hmMlspNoZMs

The beautiful paradox of fire existing (if only briefly) under water makes this video one of the most interesting things to me in geology so far.