Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like channel. More information can be found on Wikipedia here.

Pluto's Cave is a lava tube that was formed by an eruption of basaltic lava which originated from a vent about 8 miles to the northeast, between Deer Mountain and The Whaleback. Visitors can safely hike approx 1200 feet into the cave. More information here.