Wikipedia:POTD column/May 23, 2006

Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow. 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. Tubes form in one of two ways: by the crusting over of lava channels and from Pahoehoe flows where the lava is moving under the surface.

Photo credit: Michael Oswald
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