The Kurile Arc consists of at least 100 submarine volcanoes and 5 submarine caldera located mainly in the rear arc. The arc is seismically very active, particularly in the south, and is characterized by extensive volcanism and hydrothermal activity. At least one subaerial volcano on the arc (Medvezhy located on Iturup island) has an extremely shallow magma chamber and is intensely active. Three types of submarine hydrothermal deposit were recovered from this area, hydrothermal manganese crusts, nontronite and hydrothermal manganese crusts overlain by hydrogenous manganese oxides. In addition, submarine hydrothermal Fe oxyhydroxides enriched in P were sampled from the submerged caldera of Kraternaya Bight in the central part of the arc. These deposits appear to be analagous to deposits from Santorini caldera on the Aegean Arc. Piip submarine volcano located in the western Aleutian Arc is characterized by two shallow summit craters. Both intermediate-temperature (anhydrite, gypsum, barite, amorphous silica, pyrite, calcite and aragonite) and low-temperature (nontronite, Fe oxyhydroxides, hisingerite and ferromanganese crusts) hydrothermal minerals were recovered from this seamount together with bacterial mats and giant clams. The maximum measured temperature of the hydrothermal fluids was 133 °C, although the actual temperature may have reached 250 °C. Both the Kurile and Aleutian Arcs have the potential to host major submarine hydrothermal systems. Detailed exploration of these poorly studied areas using modern geophysical, geochemical and sampling techniques is therefore strongly recommended.