A hollow passage beneath the surface of a solidified lava flow, formed by the withdrawal of molten lava from a former distributory tunnel, which fed lava below the stationary surface of the flow to the advancing flow margin, sometimes transporting the lava over long distances. Tubes range in width from less than one metre to more than 30 m and in height up to 15 m. They can be tens of kilometres long, e.g. in Victoria, Australia. Most tubes are developed in flows displaying pahoehoe surface morphology, although they can develop in flows with aa surface morphology. Mount Etna, Sicily, has examples of both.