The 7.5-km-wide Tao-Rusyr caldera on southern Onekotan Island is one of the most impressive volcanoes of the Kuril Islands. The basaltic-to-andesitic caldera is filled by the deep-blue 7-km-wide Kal'tsevoe lake, whose surface is 400 m above sea level. The caldera was formed about 7500 years ago during one of the largest Holocene eruptions in the Kuril Islands. A large symmetrical post-caldera cone, 1325-m-high andesitic Krenitzyn Peak, forms a 4-km wide island that towers high above the caldera rim and fills the NW portion of the caldera lake. A 350-m-wide, 100-m-deep crater truncates the peak and a large lateral crater is located on the upper NE side. The only historical eruption of Krenitzyn Peak, in 1952, formed a small, mostly lacustral lava dome in an explosion crater along the east shore of the island.