A lavatube is a natural conduit through which lava travels beneath the surface of a lava flow. Tubes form by the crusting over of lava flows (lava may continue flowing underneath the crust for a long time, and it may be visible at syklights). When the supply of lava stops at the end of an eruption or lava is diverted elsewhere, lava in the tube system drains downslope and leaves partially empty conduits beneath the ground. Such drained tubes commonly exhibit «high-lava» marks on their walls, generally flat floors, and lava stalactites that hang from the roof. Lava can also erode downward, deepening the tube and leaving empty space above the flowing lava.
Lava tube on Kilauea, Hawaii. Photo: J. Alean