A volcano in Washington ate a river of lava and left behind a 2.5-mile cave you can explore

It’s on Mount St. Helens’ south side

Ape Cave sits on the south side of Mount St. Helens, buried under the forest floor of Gifford Pinchot National Forest. The tube runs 2.5 miles, making it the third-longest lava tube in North America.

Inside, you get zero natural light and a constant 42 degrees Fahrenheit, no matter the season. Two routes split from a single entrance, one easy and one that will test your legs.

The darkness alone changes how you move through a place.

A rare eruption hollowed out the mountain’s belly

About 2,000 years ago, a basalt lava flow ran down Mount St. Helens’ southern flank. That kind of eruption is unusual for Cascade Range volcanoes, which typically push out thicker lava.

As the outer edges of this flow cooled and hardened into a crust, the molten rock inside kept moving and eventually drained, leaving a hollow tube behind. The same event created over 60 lava tubes in the area.

A logger found Ape Cave in the late 1940s, and a scout troop explored it a few years later, naming it after their sponsor group, the St. Helens Apes.

Walk a flat sand floor in 30-foot ceilings

The Lower Cave takes you 0.75 miles on a relatively flat surface, and most people finish in about an hour. Centuries ago, a mudflow deposited sand across the floor, so the footing is uneven but manageable.

In some sections, the ceiling opens to 30 feet overhead, tall enough to feel like you’re standing inside a stone cathedral. The passage dead-ends where sand has filled in to within a couple feet of the ceiling.

Families with school-age kids handle this route well.

A 2,000-year-old lava boulder floats overhead

Halfway through the Lower Cave, you walk directly beneath a formation called the Meatball. It’s a block of cooled lava that broke from the ceiling while the tube was still full of flowing rock.

The chunk floated on the surface of the lava, drifted downstream, and wedged itself into a narrow spot above the cave floor. It has stayed there for roughly 2,000 years.

You pass right under it, close enough to study the texture of the rock above your head.

Scramble over 27 boulder piles in the dark

The Upper Cave runs 1.5 miles and takes about two to two-and-a-half hours. You climb over 27 boulder piles left behind by ceiling collapses after the lava drained out.

One section throws an 8-foot lava fall at you with only a single solid foothold, and some people need a boost to get up it.

The tube’s shape shifts constantly, from wide open passages to tight squeezes where you duck and turn sideways. Leave the young kids behind on this one.

Daylight breaks through a crack in the ceiling

About two-thirds of the way through the Upper Cave, you pass beneath the Skylight, a gap in the ceiling where daylight drops into the tunnel.

Moss and ferns ring the opening, a shock of green against black rock. The route ends at a second skylight where a metal ladder takes you back to the surface.

From there, a 1.5-mile trail winds through forest and old lava formations back to the main entrance. You walk under mature mountain hemlock, silver fir and Douglas fir the whole way.

Lava stalactites and railroad tracks line the walls

Look up inside the tube and you’ll see lava stalactites hanging from the ceiling, formed by dripping lava as the tube emptied out.

Along the walls, raised ridges called “railroad tracks” mark where lava levees built up along the flow’s edge and held their shape after the rock drained.

Terraced lines higher up work like high-water marks, recording different levels of lava that passed through over time. In places, the rock surface looks glossy where fresh lava remelted hardened stone beneath it.

All of it is basalt, common worldwide but rare for Cascade volcanoes.

Don’t touch the walls or you’ll wreck the slime

The cave walls carry a coating called “cave slime,” a nutrient-rich layer that feeds a whole food chain of tiny organisms. Your skin oils can damage the rock and throw off that ecosystem, so keep your hands off.

Crickets, grylloblattids and fungus gnats live inside the tube year-round. Bats sometimes roost here too, drawn by the stable temperature.

The cave shuts down every winter from November through mid-May partly to shield bats from White-Nose Syndrome, a fungus that has killed millions of bats across North America.

Crawl through a tunnel where a tree used to stand

Half a mile from Ape Cave, the Trail of Two Forests is a quarter-mile boardwalk over a 2,000-year-old lava flow. When that lava ran through the forest, it swallowed the trees whole.

The wood burned away and left hollow molds in the rock called lava casts.

A ladder drops into one of those molds, and you can crawl through a roughly 30-foot tunnel where a tree trunk once stood.

Above ground, old-growth Douglas fir and western red cedar grow beside younger forest reclaiming the old lava. The boardwalk is wheelchair-accessible.

Waterfalls, a mountain lake and the volcano’s south face

Farther up Forest Road 83, Lava Canyon sends waterfalls over ancient lava formations and columnar basalt. June Lake is an easy two-mile hike to a mountain lake fed by a waterfall at the base of an old lava flow.

If you want to see the volcano itself, the Lahar Viewpoint on the same road gives you the only clear look at Mount St. Helens’ south face from pavement.

Even the surface trail above Ape Cave passes small lava sinkholes and old rock formations scattered through the trees.

Bring two flashlights and forget the phone light

Ape Cave opens seasonally, typically mid-May through October.

You need a timed reservation for your vehicle through the recreation booking site, with a $2 processing fee. Parking requires a Northwest Forest Pass, an America the Beautiful Pass, or a $5 day pass you can buy on-site.

Bring at least two light sources per person because cell phone flashlights won’t cut it down there. No food, pets or smoking inside the cave.

Wear warm layers and sturdy, non-slip shoes even in July.

Total darkness and 2,000 years of volcanic force

Not many places in this country let you walk through a volcanic formation this extensive without a guided tour or artificial lighting. You set the pace.

You carry the light. The Lower Cave gives easy access, and the Upper Cave pushes your body over rock piles in the dark.

Above and below ground, you see what one eruption did to a whole landscape.

Standing in total silence inside a tunnel that molten rock carved out 2,000 years ago feels like nothing else you’ll do on a trail in the Pacific Northwest.

Explore Ape Cave in Washington

You can find Ape Cave on Forest Road 8303, about nine miles east of Cougar, Wash., inside the Mount St. Helens National Volcanic Monument. The cave sits in Gifford Pinchot National Forest in southwest Washington.

Portland, Ore., is about an hour and 45 minutes away, and Seattle is roughly three and a half hours north. The cave opens mid-May and closes in October.

Reserve your timed entry through the official recreation booking site before you go, because spots fill up.

This article was created with AI assistance and human editing.

Read more from this brand:

• Poulsbo, WA: the cozy Norwegian fishing village just a ferry ride from Seattle
• How the Cascadia Fault Could Devastate the Entire West Coast
• Amtraks first New Trains in 50 Years Are Rolling Out in the Pacific Northwest