This feature used to be a lava tube, but it since collapsed. Lava tubes can form when lava flows across the landscape. The surface cools quickly - while lava is still flowing through the center. When the source of the lava quiets down or a breach is made upstream of the tube, the molten lava in the tube keeps on flowing out, leaving a hollow. If the ceiling was thin, the tube collapses after weathering for a few years. This is a relatively small example, the Thurston lava tube is more than 4 m in diameter and more than 200 m long.