English: Professional photographer David Adam Kess to this photo at the entrance to a 2,500 meter long lava tunnel on the Island of Santa Cruz Galapagos.

Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like channel.

Lava tubes are likely to exist on previously or currently geologically active planets or moons, including the Earth.

http://en.wikipedia.org/wiki/Lava_tube