Bonefin Galaxy

Bonefin Galaxy is a galaxy found in the Wii video game Super Mario Galaxy that is covered mostly by water, with rock spires ocassionally jutting out of the ground. It is home to the antagonistic Kingfin, the boss of the level. Players will only be able to collect one star in this galaxy, though nevertheless it features a notable boss fight.

Many underwater volcanoes are placed on the ground under the sea that'll often times spew out steam.