The Maly Semyachik is a stratovolcano located in the eastern part of Kamchatka Peninsula, Russia. It is a compound stratovolcano located in a 10-km-wide caldera within the 15x20 km mid-Pleistocene Stena-Soboliny caldera. Three overlapping stratovolcanoes were constructed sequentially along a NE-SW line, with the youngest cone, Tseno-Semyachik, at the southwest end. A hot, acidic crater lake fills the historically active Troitsky Crater, which formed during a large explosive eruption of Ceno-Semiachik about 400 years ago. Its bright blue colour is due to the complex combinations of chemicals present in the water. The temperature in the lake ranges from +27 C to +42 C while the level of mineralization is equal to an average concentration of sulphate and hydrochloric acids. This amazing lake is half kilometer wide and 140 metres deep.