Lake Ilopango is a crater lake which fills a volcanic caldera in central El Salvador, immediately east of the capital city San Salvador. The caldera collapsed most recently in about A.D. 500, producing 20 times as much ash as the Mount St. Helens eruption, and blanketing an area of at least 10,000 sq km waist-deep in ash. The only historical eruption occurred in 1879, forming lava domes, now islets in the lake. Quetzaltepec is the stratovolcano just west of the city. Its last eruption in 1917 produced lavas flowing down the northwest flank, and evaporated the crater lake. Image courtesy of NASA.