There are a lot of volcanos below the surface of the sea that go down several kilometres into the ocean all around Japan.
この場合、次のような言い方ができますよ。
ーThere are a lot of volcanos below the surface of the sea that go down several kilometres into the ocean all around Japan.
「日本近海の海面から数キロメートル下に多くの(海底)火山がある」
volcanos「火山」
below the surface of the sea「海面下に」
several kilometres into the ocean「海中に数キロ」
ご参考まで!