<P> Undersea mountain ranges are mountain ranges that are mostly or entirely underwater, and specifically under the surface of an ocean . If originated from current tectonic forces, they are often referred to as a mid-ocean ridge . In contrast, if formed by past above - water volcanism, they are known as a seamount chain . The largest and best known undersea mountain range is a mid-ocean ridge, the Mid-Atlantic Ridge . It has been observed that, "similar to those on land, the undersea mountain ranges are the loci of frequent volcanic and earthquake activity". </P>

Tallest underwater mountain range in the indian and pacific oceans