Lava tube

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.

• http://en.wikipedia.org/wiki/Lava tube
• http://www.dartmouth.edu/~volcano/texts/DekHawaii.html