Lake Ilopango, Volcanic crater lake in central El Salvador.

Lake Ilopango fills an 8 by 11 km volcanic caldera in central El Salvador, on the borders of the San Salvador, La Paz and Cuscatlán departments. Its scalloped rim rises 100 to 500 m above the water, which reaches a depth of 230 m. The lake is the largest natural lake in the country.

The Tierra Blanca Joven eruption collapsed the caldera between 410 and 535 CE, though Greenland ice cores and a charred tree in the ash point to 431 CE. It ejected 37–82 km³ of material, a VEI 6 event among the largest of the last 7,000 years, and buried Maya settlements. The only historical eruption, from 31 December 1879 to 26 March 1880, built the Islas Quemadas lava dome.

At the time of the Spanish arrival, the 1532 Relación de Marroquín records Pipil settlements around the lake, including Gilopango-Tonacatepeque, Purulapa, Tepezonte and Chinuapa. Their inhabitants fished the lake for olominas, a catch still associated with its waters.

The lake lies immediately east of San Salvador, on the borders of the San Salvador, La Paz and Cuscatlán departments. Any overflow drains through the Jiboa River toward the Pacific. Divers at Cerros Quemados reach depths of 27 to 30 m, where an underwater sand beach lies. The Ilopango international airbase holds annual airshows with pilots flying over the lake.

The only historical eruption of the caldera, lasting from 31 December 1879 to 26 March 1880, built a lava dome that reached the surface and formed the Islas Quemadas. The same event raised the lake level and caused landslides and cracks in nearby hamlets. Divers can now reach the dome at Cerros Quemados, where an underwater sand beach lies at 27 to 30 m.