Sentinel Cave runs about 1,000 metres (3,280 feet) under Lava Beds National Monument in northern California, and it is one of the few developed caves in the park with two entrances, marked as upper and lower Sentinel on the park map, so you can walk in at one end and come out at the other. The park counts it among its least challenging caves: the main trail asks for no stooping or ducking, though it climbs and drops on stairs along the way.
It is really four caves. The USGS describes Sentinel as the ruins of four large lava tubes stacked one above another, the lowest slightly offset, all formed by flows of smooth lava between 10,500 and 65,000 years ago. After the volcanism ended their roofs and floors partly collapsed, tumbling into a chaos of big angular blocks, and in many places a light played across the ceiling shows only the gaps where blocks have fallen out. Loose blocks still perch overhead, and collapse is still under way. The uppermost tube was the main lava feeder of the Cave Loop area, linking Natural Bridge to Ovis, Indian Well and Sentinel caves, and Sentinel's roof is the largest stretch of it left standing.
The walk shows the ruin from the inside. A natural catwalk, anchored at each end to the old floor of the upper tube, crosses deep collapse pits, with a balcony of lava three feet above it; elsewhere the trail climbs a huge mound of rubble and descends a stairway into a collapse where the whole upper floor has fallen into the tubes below. The main passage reaches 10 feet wide and as much as 25 feet high. A double skylight called the Tube and Pillar entrance lights the middle level, and a gated, chimney-like pit drops to the lowest levels.
Beyond the lower entrance, 66 feet below the upper one, the cave changes character. Through the Annex entrance a 15-foot tube runs on with its walls and floor largely intact, moist and nearly black, much colder, with ropy pahoehoe underfoot and peeled linings on the walls, until it is sealed by lava 106 feet below the upper entrance. The park keeps the deeper passages for experienced cavers. Lava Beds shelters at least 14 species of bats, and rangers ask anyone who meets them to turn their light away and move on quietly.
Other places on the map within a couple of hours.
