On the island of Terceira an extraordinary volcanic phenomenon happened, one of the most spectacular of the Azores. The Algar do Carvão is located within the Caldeira Guilherme Moniz and is accessible to visitors. It developed in the form of a volcanic cone. Inside access is through an artificial tunnel and a staircase, where we can see the route taken by the river of lava ejected by the volcano but hampered by a thick wall of stone. The highlights are the underground lake and the set of stalactites and stalagmites of volcanic glass.