lava tube

A natural tunnel formed when the surface of a lava flow hardens into a crust while molten lava continues to flow beneath it. When the eruption ends and the remaining lava drains out, it leaves behind a hollow tube — sometimes miles long, sometimes large enough to walk through upright. Lava tubes are found on every volcanic island and shield volcano, and they have been identified on the Moon and Mars. They are the earth's own plumbing, built by fire and left as caves.