Algar do Carvao

Algar do Carvao is a volcanic cave on Terceira Island in the Azores, Portugal, formed within the Pico Alto volcanic system as a combined volcanic chimney and lava tube. After an eruption fed magma up through the vent, much of the molten rock later drained back downward, leaving behind a near-vertical shaft roughly 45 meters deep opening into a wide underground chamber. The lower part of the cave is flooded by a small lake, and its walls are lined with unusual stalactite-like mineral formations built from silica rather than the calcium carbonate typical of limestone caves, a rare feature that makes it one of the few volcanic caves in the world known for this kind of speleothem.