lava tube

Lava tubes are geological formations created by flowing lava, primarily from basaltic types like pahoehoe. These caves form when the surface of a lava flow solidifies, creating a roof over the molten rock beneath, which continues to flow and drain away, leaving behind a tunnel.12

These tubes are typically long and can be complex, often oriented downslope from a volcanic vent. Their roofs and walls are made of solidified lava, and their floors may be covered with sand or other debris. Lava tubes are usually found in recent lava flows, less than 20 million years old, as they are easily destroyed by erosion. They are distinct from caves formed by the dissolution of bedrock, such as limestone caves.12