Lava Tube

A lava tube, occasionally called a pyroduct or lava tunnel, is a natural roofed conduit through which molten lava flows away from a volcanic vent. If the lava inside drains away, it leaves behind an empty cave. Lava tubes are common in low-viscosity volcanic systems and can carry molten lava much farther from the eruptive vent than an open lava channel, since the solid crust that forms over the tube shields the flowing lava inside from atmospheric cooling.