Lava Tube

Caves at Craters of the Moon represent Pleistocene or Holocene basaltic lava tubes. The number of documented caves currently exceeds 300, and with further exploration and inventory a realistic estimate of the number might exceed 600. Along the Great Rift, the volcanic rift zone in which the lava fields formed, are additional cave features termed "fissure caves". These fissure caves are present in much fewer numbers than the lava tube caves. Some of these fissures are quite sizeable, including one particular fissure that may be passable to a depth of 650 feet (200 meters) from the surface.

The Virtual Lava Tube
This web site provides illustrations of a variety geologic features found in lava tubes.