Ape Cave is a 2-mile-long lava tube formed 2000 years ago by a lava flow that followed a deep watercourse. It's the longest lava tube in the western hemisphere. Hikers can walk and scramble on either the 0.8-mile Lower Ape Cave Trail or the 1.5-mile Upper Ape Cave Trail, which requires some navigating over rock piles and through narrower passages. A Northwest Forest Pass ($30/5 per year/day) is required to park at the trailhead.
The trail eventually exits at the upper entrance. You can bring your own light source or rent lanterns for $5 at Apes' Headquarters, located at the entrance to the caves. In summer, rangers lead free explorations of the trails several times a day.
Another interesting side trail is the 0.6-mile, wheelchair-accessible Trail of Two Forests, on a boardwalk across a 1900-year-old lava flow that once buried an ancient forest.