There’s a 2.5-Mile Lava Tube Under Mt. St. Helens with a “Meatball,” Cave Slime, and Amazing Hikes

Washington’s Ape Caves

Washington’s longest lava tube snakes beneath Mount St. Helens, dark and cool year-round. The cave system splits into two routes: a chill lower section for casual walks, and a more challenging upper tube where you’ll need to climb over rock piles and duck under low spots. Here’s more about the Pacific Northwest’s wildest underground hike.

Ape Caves Is North America’s Third Longest Lava Tube

The Ape Cave isn’t a typical limestone cave. It’s a lava tube stretching 2.5 miles, making it North America’s third-longest of its kind.

Located on Mount St. Helens’ south side in Gifford Pinchot National Forest, this underground passage is a 13,042-foot-long record of volcanic activity. The cave’s smooth walls and high ceilings reach up to 50 feet in some spots.

The Name Comes From A Boy Scout Group

The cave’s name has nothing to do with Bigfoot or real primates. It was discovered in 1947 by logger Lawrence Johnson, and its exploration began in the early 1950s by a Boy Scout troop led by Harry Reese, who named it after their sponsor, the St. Helens Apes.

Lava Created These Tunnels 2,000 Years Ago

About 2,000 years ago, basaltic lava flowed down Mount St. Helens’ southern side. The outer layer cooled and hardened when it touched air, making a solid crust. Meanwhile, hot lava kept flowing inside this natural pipe, eventually draining out completely and leaving the hollow tube we explore today.

Scientists have traced the lava to the Hopeless and Ape Canyon vents on the mountain’s flank. The smooth walls formed as flowing lava repeatedly melted the inner surface.

Hiking the Upper vs. Lower Cave

You can choose between two different experiences at Ape Cave. The Lower Cave runs about 0.75 miles and is fairly easy to walk through, perfect if you’re bringing kids or trying caving for the first time.

The Upper Cave offers a more challenging 1.5-mile journey with boulder piles, an 8-foot lava fall, and narrow spots you’ll need to scramble through. Unlike the Lower Cave where you return the way you came, the Upper Cave route exits through a forest trail that loops back to the main entrance.

There’s a “Meatball” Inside Ape Cave

One of the most popular sights inside Ape Cave is the “Meatball” formation in the Lower Cave section. It is a block of cooled lava which fell from the lava tube ceiling while lava was still flowing through the cave.

Despite how it looks, the Meatball has stayed firmly in place for 2,000 years. It creates a narrow passage beneath it that you’ll need to navigate, giving you an up-close look at this unique feature. Rangers often use the Meatball as a landmark when giving directions inside the cave.

There’s Also a Skylight

Halfway through the Upper Cave, you’ll find the “Skylight,” a surface opening in the ceiling that lets daylight into the otherwise dark tunnel. This opening wasn’t part of the original tube but over time, sections of the hollow tube collapsed from above.

The Skylight allows fresh air into the cave system and creates a striking scene where light meets darkness. During winter, snow sometimes falls through the opening, adding an even more magical feel to the cave.

You Need To Climb An Eight Foot Wall Inside

The toughest challenge in Ape Cave is the “Lava Fall,” an 8-foot wall of hardened lava in the Upper Cave. This feature formed as the lava flowed, the outer edges began to cool and solidify, forming a hardened crust.

To continue through the Upper Cave, you need to climb this wall. You won’t need climbing gear, but you will need some strength and careful footing. Many people say this challenge is their favorite part of visiting the cave, with the best sections waiting just beyond.

The Living Cave Slime

The cave walls host a delicate ecosystem often called “cave slime.” This thin layer consists of bacteria, fungi, and other tiny organisms that have adapted to the dark, cool environment.

These microbes get nutrients from minerals in the rock and organic material that comes in with air and visitors. The biofilm looks like shiny patches on the walls in colors from gold to purple. Don’t touch these, because touching kills cave slime, a basis for the food chain of tiny creatures that live there.

The Cave Stays Around 42°F All Year

No matter if it’s 90°F outside or below freezing, Ape Cave stays at 42°F (5.6°C) all year. This steady temperature is typical of deep caves, which stay insulated from outside weather. The stable climate helps preserve the cave’s features and supports its unique ecosystem.

Many people are surprised by how quickly it cools down when entering the cave, especially in summer when the difference can be over 40 degrees.

Check Out the Tree Casts While You’re There

The area around Ape Cave has plenty of other cool spots to check out. Just a short 4 minute drive away, the Trail of Two Forests shows “tree casts.” These are hollow tubes formed when lava flowed around trees, which later burned away, leaving perfect molds of trunks and roots.

You can also explore other parts of Mount St. Helens, including views of the 1980 eruption area. Many people combine their cave trip with stops at Lava Canyon, June Lake, or the Johnston Ridge Observatory for a complete volcanic experience.

Visiting Ape Cave

From May through October, you must make reservations on recreation.gov. Bring at least two separate light sources (headlamps work best), but preferably three light sources, since the cave has no lighting.

Wear sturdy closed-toe shoes with good grip and pack warm clothes regardless of the outside weather. The best time to visit is midweek during spring or fall when there are fewer people.

Forest Service rangers also lead tours during summer that can teach you things you might miss on your own, and we highly recommend joining one.