A lava tube can form when a flowing lava stream develops a solid roof while molten rock continues moving underneath. If that lava drains away, a passage remains. This Mojave photograph shows daylight entering through an opening in the roof.
The useful clue is the relationship between a cave and the lava flow around it. Its origin differs from limestone caves enlarged by dissolving rock. Tubes can branch or contain multiple levels; the single opening shown here does not reveal the entire passage.
Source / photo: NPS · Public domain. Resized. No claim to original U.S. Government works.