Troitsky crater, the youngest of six craters at the summit of Kamchatka's Maly Semiachik volcano, was formed during a major explosive eruption about 400 years ago. The crater, seen here from the west with the Pacific Ocean in the background, truncates the snow-covered slopes of Ceno-Semiachik, the youngest of the four overlapping stratovolcanoes that comprise the Maly Semiachik massif. The crater, now filled by a hot, acidic lake, has been the source of all historical eruptions from the volcano.
Photo by Dan Miller, 1990 (U.S. Geological Survey).