The Archipelago of the Azores is situated in the middle of the North Atlantic Ocean, at latitudes 37° and 40°N， longitudes 25° and 31°W, about 1600 km from the European Continent. The Azores consists of nine islands, which all are of volcanic origin. There have been about 30 eruptions since the islands were settled in the 15th century. Terceira Island is the third largest Island of the Archipelago, with an area of 402 square kilometers. There are three calderas and many lava domes and pyroclastic cones on this island. Algar do Carvão is a cave in a scoria cone in the central part of Terceira Island. This cave is equivalent to the volcanic conduit of a cone, with a surface diameter of 20 m and depth of 90 m. The entrance is located at a depth of 30 m (where two persons are located). Descending the stairs, visitors find many large connected chambers and a pool at the bottom. They feel as if they are undertaking an underground expedition.