West Mata underwater volcano is currently the deepest erupting submarine volcano in the ocean, and the first to ever be filmed erupting. The volcanic eruption, discovered in May 2009, is nearly 1,2 km below the surface of the Pacific Ocean, in an area bounded by Fiji, Tonga and Samoa.
This video shows a sequence of explosive degassing events as bubbles of gas-rich magma burst, spewing lava fragments into the water. At the same time, lava is being extruded out of the vent and down the slope. The area in view is about 2-3 m across in an eruptive area approximately 90 m that runs along the summit.
News story: Marine scientists discover deepest undersea erupting volcano