The archipelago is formed by high points on the rim of the caldera of a submarine volcano that forms a seamount. The volcano is one part of a range that was formed as part of the same process that formed the floor of the Atlantic, and the Mid-Atlantic Ridge. The top of the seamount has gone through periods of complete submergence, during which its limestone cap was formed by marine organisms, and during the Ice Ages the entire caldera was above sea level, forming an island of approximately two hundred square miles.