sentences:  Lava tubes are formed when a low-viscosity lava flow develops a continuous and hard crust which then thickens and forms a roof above the molten lava stream.When the eruption subsides, the still-molten lava moving beneath the crust will continue to drain downhill, leaving an open lava tube.Many other lava tubes have been discovered in Hallmundarhraun, most notably Surtshellir and Stefánshellir.