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Orbital photographic and remote sensing surveys of the Moon and Mars both show extensive evidence of lava tube formation. These natural caverns form as the result of lava flows that have overcrusted to form subsurface flowing rivers of lava; as they drain an open conduit is left behind. The scale of these caverns can be inferred from the related sinuous rilles which represent lava tubes that have collapsed and nearby craters which have clearly defined topographic features. From this we estimate that the dimensions of lunar tubes are much larger than terrestrial lava tubes. The size of these rilles and associated topographic ridges (which may represent sections of a tube that have not collapsed) suggest cross-sectional widths on the order of hundreds of meters, lengths of tens of kilometers, and roofs that are meters thick.