Firefighters use water to battle a towering inferno. You might wonder if water could also stifle an erupting volcano. Actually, this experiment happens every time a volcano erupts at the bottom of the ocean. What Happens When A Volcano Erupts Underwater? Most of the world’s volcanic action happens an average of eight thousand five hundred feet underwater. Deep beneath the waves there are an estimated one million volcanoes. As with land volcanoes, molten lava pushes up from beneath the sea floor at thousands of degrees Fahrenheit.