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Lava Tube Cave

Lava-tube caves, like this one in California's Lava Beds National Monument, are found throughout the world. These unique underground structures form during long-lasting lava flows. As lava moves through a channel, overflows build natural levees along the sides, which can eventually connect and harden, forming a canopy. Just as a winter stream continues to flow beneath its ice cap, lava continues to move under this roof. When flow from the source stops, the remaining lava moves through to the end, leaving a hollow tube, often large enough to walk through.

Photograph by Tim Laman