Lava tube

A natural tunnel formed when the surface of a lava flow hardens while molten lava continues moving beneath it. After the flow drains, the hardened roof encloses an empty passage.

Lava flow: Molten rock moving across a planet’s surface during an eruption. A lava tube develops within a flow whose outer surface cools before its interior.
Basalt: A dark, fine-grained volcanic rock formed when low-silica lava cools. Most well-known lava tubes form in basaltic flows.
Lava tube cave: A cave formed within a lava flow after molten lava drains from an enclosed passage. The empty cave is the accessible form of a drained lava tube.
Kazumura Cave: A lava tube system on Hawaiʻi Island, recognized as one of the world’s longest known. Its extensive passages show how far a tube system can extend within a basalt flow.
Lunar pit: A steep-walled opening on the Moon, often attributed to collapse or volcanic processes. Some lunar pits may be skylights into lava tubes, but their subsurface extent is uncertain.
Crust formation: The formation of a solid outer layer as molten material cools. Cooling lava forms the roof and walls that confine the moving melt.
Lava: Molten rock that has erupted onto a planetary surface. A tube begins as lava moving beneath a cooling surface.
Lunar lava tube: A subsurface passage on the Moon inferred to have formed beneath a lava flow. Lava tubes are proposed shelters from radiation and micrometeorites for lunar exploration.
Nāhuku: A lava tube in Hawaiʻi Volcanoes National Park, also known as Thurston Lava Tube. Its accessible passage offers a familiar example of a tube shaped by flowing lava.
Planetary geology: The study of the composition, structure, and history of planets and other solid worlds. Comparing candidate tubes tests whether terrestrial formation models apply on other worlds.

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