San Juan Seamount: An Ancient Archipelago

Some seamounts however, including the San Juan Seamount off the coast of southern California, were once ancient islands.
For around a million years before it slipped back beneath the waves (10-14 million years ago), it existed as a series of eight small islands, maxing out at 460 feet above sea level with a total area of approximately one square mile (a little larger than Monaco and Vatican City – the world’s smallest countries).
Today, the summit of San Juan seamount sits 1,837 feet below the surface of the ocean.
In the case of San Juan Seamount, the ocean crust also compressed significantly due to thermal contraction – meaning that the crust shrank as it cooled down once volcanic activity ceased.
Deep-Sea Exploration San Juan Seamount was first explored in 1941 when a single dredge recovered rocks from a depth of 5,000 feet (two other dredges were attempted, of these one was damaged and the other lost).
In 2003 and 2004, the Monterey Bay Aquarium Research Institute conducted the first visual surveys of the seamount using a considerably more advanced approach, subsequently followed by a 2016 expedition by Ocean Exploration Trust.
Remotely operated vehicles (ROVs) are robotic submersibles that are piloted from a ship.
Like HOVs and ROVs, these vehicles can visually survey the seafloor and carry a multitude of sensors.
However, AUVs carry out their surveys without being piloted by a human, operating according to a pre-programmed mission.
AUVs can generally carry out longer missions and cover much greater areas of the seafloor than ROVs and HOVs, but have reduced sampling abilities and cannot navigate highly complex environments.