Ushishur volcano in the central Kuriles has one of the world's most beautiful calderas, forming a almost complete ring rising above the sea. Most of the volcano's edifice is under water, but the submerged part forms 2 small islands. The southern island contains the summit caldera and the northern a portion of the volcano's flanks. The small 1.6-km-wide caldera of the southern island formed about 9400 years ago and has only a narrow breached on the south, allowing sea water to fill the caldera. Eruptions after the formation of the caldera formed small islands in the caldera bay.

2 younger andesitic lava domes occupy part of the caldera bay. 2 other older domes are joined by a sand bar to the SE caldera wall. The 2 younger domes erupted sometime after the 1769 visit of Captain Snow to form islands in the bay.

A cluster of strong fumaroles and hot springs along the SE caldera shoreline was a sacred place to 18th- and 19th-century Kurile Ainu peoples. Vigorous submarine hydrothermal activity has modified the geochemistry of sea water within the caldera bay.

Aside from growth of the 2 younger lava domes, only minor phreatic eruptions have occurred at Ushishur during historical time.

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Source: Smithsonian GVP Ushishur volcano information