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As the outermost portion of the hot lava flow comes into contact with the cold air, it cools rapidly, forming a hardened crust, explains Dr. Richard Léveillé. Léveillé is an adjunct professor at º«¹úÂãÎè's Department of Earth and Planetary Sciences and a member of the º«¹úÂãÎè Space Institute. 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. Boom — a lava tube is born.