What are seamounts?

Seamounts are undersea mountains (usually of volcanic origin) rising from the seafloor and peaking below sea level. Underwater mountains of heights above 1000 m are considered to be seamounts, those between 500-1000 m as knolls, and those below 500 m as hills. A seamount tall enough to break the sea surface is called an oceanic island, e.g., the islands of Hawaii, the Azores and Bermuda were all underwater seamounts at some point in the past.

Photo of a deep-sea rattail fish and seastar
Courtesy of Lisa Levin/ONR