Marsili is a large undersea volcano in the Tyrrhenian Sea, about 175 kilometers (109 mi) south of Naples. The seamount is about 3,000 m (9,800 feet) tall; its peak and crater are about 450 m below the sea surface. Though it has not erupted in recorded history, volcanologists believe that Marsili is a relatively fragile-walled structure, made of low-density and unstable rocks, fed by the underlying shallow magma chamber.