Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow. Typically they are expelled by a volcano during an eruption and can be actively draining lava from a source; or they can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like channel.
Lava tubes are a type of lava cave, formed when an active low-viscosity lava flow develops a continuous and hard crust, which thickens and forms a roof above the still-flowing lava stream. Tubes form in one of two ways: by the crusting over of lava channels, and from pahoehoe (lava) flows where the lava is moving under the surface.
Lava tubes can be up to 14–15 metres (46–49 ft) wide, though are often narrower, and run anywhere from 1–15 metres (3 ft 3 in–49 ft 3 in) below the surface. Lava tubes can also be extremely long; one tube from the Mauna Loa 1859 flow enters the ocean about 50 kilometers (31 mi) from its eruption point, and the Cueva del Viento – Sobrado system on Teide, Tenerife island, is over 18 kilometers (11 mi) long, due to extensive braided maze areas at the upper zones of the system