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".