Lava tubes are common on Pico. They form when a stream of hot lava finds its way across cooler rock. Drippings often happen at the ceiling, forming stalactite-like formations called lavacicles (image 5). Sometimes, a hanging floor forms as an after-effect of level change inside the tube (image 7). New flows also happen (image 4). The pictures below were taken at Furna de Frei Matias. 

Gruta das Torres is a big lava tube 3 miles SE of Madalena. It can be visited on a few hundreds yards, a fraction of the total of 3.2 miles (5.2 kilometres).