This is a short hike through a lava tube just south of Mount Saint Helens in Gifford Pinchot National Forest (Washington state). The cave is very dark – two light sources per person are recommended. There’s not terribly much to see, but it’s a unique and interesting experience. Including the return hike, the trip is under 3 miles. A parking permit is required and can be purchased at the parking lot. During the summer, lanterns can be rented at the kiosk near the entrance.
Directions to Trailhead: From the town of Cougar, WA, drive East on Rd 90. Turn left at Rd 83, then left again at Rd 8303. The cave parking lot is only about 20 minutes from Cougar. It is well signed and easy to find.
For more info, check out the Ape Cave Norcalhiker blog post.