Lunar lava tubes are sub-surface tunnels on the Moon that are believed to have formed during basaltic lava flows. When the surface of a lava tube cools, it forms a hardened lid that contains the ongoing lava flow beneath the surface in a conduit-shaped passage. Once the flow of lava diminishes, the tunnel may become drained, forming a hollow void. Lunar lava tubes are formed on surfaces that have a slope that ranges in angle from 0.4–6.5°. Lunar lava tubes may be as wide as before they become unstable against gravitational collapse. However, stable tubes may still be disrupted by seismic events or meteoroid bombardment.