These caves often form out of a type of fluid, basaltic lava, that flows down a slope like the side of a volcano. As the outermost portion of the hot lava flow comes into contact with the cold air, it cools rapidly, forming a hardened crust. But liquid lava continues to flow like water in a channel underneath this newly hardened surface. At some point, that liquid lava runs out and cools underneath the surface, forming a curvy, tube-shaped structure.