Ape Cave is a world-class underground adventure on the south flank of Mount St. Helens in Washington's Gifford Pinchot National Forest — the longest lava tube in the continental United States, stretching more than two miles through the dark heart of a 2,000-year-old lava flow. A trip into this cool, pitch-black tube is one of the most unique and memorable outings in the entire Mount St. Helens National Volcanic Monument.

Visitors can explore two routes: the easier Lower Cave (an out-and-back walk on a relatively smooth floor) and the more challenging Upper Cave, a 1.5-mile route that requires scrambling over rock piles and climbing an eight-foot lava wall, exiting via a forest trail back to the start. Either way, it's an awe-inspiring journey through a volcanic landscape sculpted by molten rock.

Come prepared — this is a real cave, not a lit tourist attraction. There are no lights inside, so you must bring at least two reliable light sources per person (headlamps are ideal; lanterns can be rented at the on-site headquarters in summer). The cave stays a chilly 42 degrees year-round, so bring a jacket and sturdy, closed-toe shoes with good grip.

Plan ahead for access: in the summer season (roughly May through September), timed-entry reservations are required and must be purchased in advance through Recreation.gov (a small fee, one per vehicle) — they're not sold on-site. Help protect the cave's bats by not bringing gear worn in other caves. Bring your lights, layers, and sense of adventure, tread carefully on the rocky floor, and Ape Cave delivers an unforgettable walk through the underground world of a volcano.