The Summitville mine was the scene of a major environmental disaster in the 1990s when the liner of a cyanide-laced tailing pond began leaking heavily. Summitville is in the Summitville caldera, one of many extinct volcanoes making up the San Juan volcanic field. One, La Garita Caldera, is in diameter. Large beds of lava, some extending under the floor of the San Luis Valley, are characteristic of the eastern slope of the San Juans.