The primates that gave their name to two lava tubes found along this trail weren't monkeys-they were members of a 1950s outdoor club who found and explored the tubes. They called themselves the Mount St. Helens Apes, and the lava tubes became known as their caves. The tubes are long tunnels in the thick lava beds; they run roughly parallel to the surface of the land. Interpretive signs line both the trail through the forest and the tubes' mouths. The lower tube is the easiest (but still requires a certain amount of care) and the upper tube is larger. It is not possible to hike in the caves the entire length between the two entrances. Descending into the tubes requires a jacket-it's a constant, cool 42 degrees under the earth, regardless of what happens on the surface-and a powerful flashlight or lantern. The tube beds are rough and uneven.

Note: Powerful flashlights with well-charged batteries or a strong lantern are required for walking in the caves. Do not try to explore these spots without a good light.

A pleasant, flat 1.3-mile trail through the old forest links the two lava tubes and leads from the trailhead to these underworld entrances. This trail leads through wonderful old forests. About 1 mile out, the trail passes a small crack in the ground. This "skylight" allows hikers to peer into the caves and allows cave explorers to see a bit of sunlight. The trail ends at the upper cave entrance.