Created by the volcanic forces that once rent that section. The largest of the caves extend about one mile under the mountain to an opening on another face. The caves contain no stalactites or stalagmites, as the formation is lava, not limestone. Flashlights or lanterns should be used when entering them, and tourists should be accompanied by someone familiar with the caves. Sturdy, thick-soled shoes and other suitable clothing should be worn.

Many caves exist in Washington, and most of them are lava tubes, produced by lava streams from the big volcanoes. These lava tubes are among the longest in the world.