The 12,810-foot Ape Cave lava tube is one of the longest such caves in the nation, and one of the most popular visitor attractions at Mount St. Helens. The cave is 10 miles northeast of Cougar at the junction of Forest Roads 83 and 90 and is open all the time.

From the entrance a staircase leads down to a chamber where the route splits. The downhill arm ends after an easy 0.75-mile walk, while the uphill route is more difficult and rocky, continuing 1.5 miles to an exit where an above-ground trail leads back to the starting point.

Forest Service interpreters lead half-hour tours of Ape Cave twice a day in the summer. Be sure to bring drinking water and two flashlights and extra batteries (or rent a lantern for $2). Wear hiking boots, gloves, and warm clothes (the air is a steady 42°F all year).

A small visitors center (daily 9 a.m.–6 p.m., mid-June through Labor Day) can provide assistance, publications, and lantern rentals. A Northwest Forest Pass ($5) is required for access to the cave. The road is plowed to the Trail of Two Forests, a half mile away, in the winter.