Ape Cave

Ape Cave is the third longest contiguous lava tube (2.5mi long) in the North America. Located at the base of Mount Saint Helen, the cave is broken in two sections: Ape Cave is extremely popular on weekends, particularly the lower section. The Forest Service administers Ape Caves as a public attraction, consequently you're likely to find many underprepared or new cavers as well as trash and inappropriate behavior. This might be a good opportunity for some friendly education. The cave is fabulously long; even experienced cavers might enjoy the experience. Come mid-week or in winter for more solitude. The main Ape Cave Parking lot is generally closed in the winter season, so you'll need to park at the nearby Trail of Two Forests and hike or snowshoe in. (Update: cave may have been gated which makes winter entry impossible.)

From the parking area, you have two choices:

The caves are touristic but non-commercial, so bring several light sources, gloves, and a helmet. Its advantageous to bring a powerful light. The dark lava absorbs more light than light-colored limestone caves. Be prepared for cold, the cave stays at constant temperature of 42F even in summer. You have to climb a few small lava waterfalls in Upper Ape Cave. They are fun, and the lava provides good footing, but can be difficult for small children.