Underwater volcanos or Submarine volcanos are mostly found as underwater mountains. They are formed from extinct volcanoes. There are many of these underwater volcanoes. Most of them will not form to make it to the surface. It is also thought that they have a 75% magma output and when the volcano erupts it can make the surrounding water pressure 250 times more than normal. Amazingly sea life can live and thrive close by these volcanoes.