noun
submarine volcano
A volcano located on the ocean floor.
海底火山の噴火で新しい島ができた。
A new island was formed by the eruption of a submarine volcano.
火山 is a general term for any volcano, while 海底火山 specifically refers to volcanoes located on the seafloor.
Compound of 海底 (seabed) and 火山 (volcano).