Furna do Enxofre - Sulphur Cavern - is a large cave of volcanic origin located on the Graciosa Islanda, Azores.
This place is one of the portuguese sites on the UNESCO Tentative List.
Photo by Jorge Góis
It's a huge cave located in the only caldera of the island, whose origin is related to the collapse phase and drainage of a lava lake in the interior of the caldera. It is formed by an unique and huge dome that shows the existence of basaltic prisms. its maximum altitude is 100m and its diameter is 150m. In the bottom of the cave a big lagoon can be seen. There is a permanent fumarolic activity which exhales the CO², H²S, H² gases There are two holes in this cave that allow the entrance of the sun light. - in: http://whc.unesco.org/en/tentativelists/566/