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In Lava Beds National Monument (Northern California), this is the Mushpot Cave trail, which is a cave created when red hot lavas flowed underground. Once the lava drained away, this lava tube cave was left behind. There are over 500 such caves known to exist in Lava Beds National Monument (some are over a mile long), with new ones discovered each year. Looking up at the roof of the cave, this is called a "roofed-over skylight." Skylights are holes in the ceiling that open to the surface, like the opening at the entrance to this cave. This particular skylight was changed when a lava flow filled the tube here and pushed up into the hole. The lava crusted over within the skylight, and when the flow subsided, it left behind a dome of hardened lava. (09/29/2004)