1. Manjanggul Lava Tube – Jeju Island, South Korea
Stunning Pictures of Lava Tubes
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. [Source]
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. [Source]
Lava tubes can be up to 14–15 meters (46–49 ft) wide, though are often narrower, and run anywhere from 1–15 meters (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. [Source]