Boulder Cave Day Use Site is open April 1 to October 31. The area is managed by Northwest Land Management. There is a $5 fee for parking.

This hike will be a thrill for children. Make sure to bring a flashlight — or two — and prepare to do some spelunking.

There is a picnic area near the trailhead that includes a group shelter with a large stone fireplace built by the Civilian Conservation Corps in 1935.

Drinking water is available among the picnic tables and fire rings.

Make sure to read the information at the trailhead kiosk. There is a wealth of information posted here, such as the fact that Boulder Cave is the largest of its kind in North America, and that it was discovered by prospectors on Aug. 2, 1901.

Unlike most caves, this is not a limestone cavern or a lava tube.

It took millions of years of volcanic action to create the landscape, and thousands of years for erosion and weathering to shape the features seen in the 200-foot long cave.

Over the years, lava flows making up the Yakima Basalt Formation covered deposits of soft, loose rock and soil. When the lava cooled, it trapped the layers of loose sediment.

Then came the water, which over thousands of years has etched its signature into the basalt.

The water flowing in Devil’s Creek eroded a deep channel through the top layer of hard basalt and the softer layer underneath. When Devil’s Creek met the next layer of hard basalt, the creek found the path of least resistance and began to erode the softer deposits on a horizontal plane. In time, that erosion carved a hollow pocket underneath the top layer of basalt.

About 25,000 years ago, the outer edge of this pocket weakened and collapsed, leaving an archway 350 feet long and 30 feet wide. And Boulder Cave was born.
Devil’s Creek continues to flow through and expand the cave it created.

Boulder Cave and Devil’s Creek look very much like they did thousands of years ago.

Over the years, bats have taken up residence in the caves. Boulder Cave houses the only known population of Pacific western big-eared bats in this part of Washington.

In the 1920s and 1930s, the Boulder Cave bat population numbered in the thousands. The bats once used Boulder Cave during the summer, but they no longer do.

It appears heavy human use during the summer has discouraged the big-eared bats from sticking around through the summer months.

Closing the cave in the winter helps protect the bats that do return.

Today, about 50 hibernate in Boulder Cave.

Location: About 47 miles west of Yakima

Distance: 2 miles

Destination: Out and back

Difficulty: Easy

Elevation gain: 200 feet

Getting there: Boulder Cave is near milepost 95.43 on State Route 410, about 47 miles west of Yakima. From State Route 410, turn north on Forest Service road No. 1706. Cross a bridge. Turn right toward Camp Roganunda. Watch for signs leading to Boulder Cave.

The Trek: Pick up the trail to the left of the kiosk. It climbs a short hill and enters a forest teeming with such conifers as ponderosa pine, Douglas fir, western larch and grand fir. A sign explains how to tell the various species apart by taking a close look at their needles and needle clusters.

Continuing uphill, the trail climbs as it hugs the canyon carved out of the surrounding lava flows by Devil’s Creek.

A short detour to the left leads to another sign, this one explaining how the area’s fire history can be seen on the trees. Tree rings not only tell age, rate of growth, wet and dry years, but also where fire raged through the area.

Ponderosa pine, Douglas fir and western larch have adapted to fire by having thick, fire tolerant bark and by having branches high off the ground.

Continuing on, reconnect with the main trail as it continues within the canyon. It rises and falls as it follows the course of the creek far below.

A trail to the left leads to a precarious overlook. The large wood deck extends out over the canyon, providing great views up and down the creek-cut gorge. On both sides of the stream, walls of rock steeply slope to the gorge’s floor. Trees, looking like broken matchsticks, litter the steep inclines.

A sign highlights the plant life that calls this area home. It explains how Oregon grape, yarrow and vanilla leaf were used by American Indians. For instance, Oregon grape was used as food, dye and medicine.

Attached to the overlook’s far rail is another sign, this one explaining that another, younger cave is beginning to take shape just upstream from where you are.
Take the trail to the left of a fallen tree to return to the main trail, then continue left.

It is a short distance before the “young cave” can be seen. It’s only a matter of time — geologic time that is — before it grows in size. Cracks in the steep canyon walls are already forming. Eventually, the unsupported weight of the cave ceiling will cause these fissures to separate and a massive basalt slab will crash to the valley floor, once again blocking Devil’s Creek.

The path passes several rock outcroppings, which precariously teeter from the main face. It’s here that I encountered a chipmunk with his cheeks stuffed with food.
Bending to the left, the trail continues along the edge, passing stands of ponderosa pines. Clearings now allow for better views of the younger cave. It has the look of a large, gaping mouth about to take a bite out of the stream.

Fluctuating between double and single track, the trail soon reaches a “Y.” Stay to the left.

After a short descent, reach another “Y.” This one has a sign between the two routes. After reading more about Boulder Cave, head left. The right will be the return route.

Switchback to the gorge floor, passing plenty of downed trees and massive basalt walls. Near the bottom, the trail becomes a series of boardwalks between overlooks.
The first overlook provides a view upstream to the mouth of Boulder Cave. It also has a sign explaining the differences between moss and lichen.

Continue right to reach another observation platform. This one allows a glimpse upstream and is a better vantage point for viewing Devil’s Creek.

Return to the path and proceed left, descending to creek level on a path of crushed rock. As you walk through a fissure carved by the stream, one gets a sense of just how massive these basalt flows are. Piles of broken rock border the trail.

Reach another section of boardwalk and round a bend. Blue sky can be seen through a gap in the rocks.

Get the flashlights out and make sure they work. Once you are set, head into the cave. Light fades quickly as you enter the massive void. Try not to think about the tons of rock directly above your head.

It gets very dark very quick. Be prepared for the plunge into darkness. Keep your flashlight focused on the trail and speak in hushed tones just in case there are bats hanging out in the cave.

The path winds around more rocks, and there are plenty on the trail.

Continuing on, the cave opens up as the ceiling is now well above your head. On the other side of the stream, the rock disappears into the ground.

Eventually, light begins to filter in as the cave’s exit is reached. Here, Devil’s Creek rumbles over rocks, forming a series of small waterfalls. Continue toward the light.
Pupils dilate quickly as one goes from ink-blackness to bright daylight.

There is not a bat to be seen or heard on this day.

Outside the cave’s mouth, Devil’s Creek steals the show as it makes its way toward the cave by way of waterfalls and plunge pools.

Continue to wind your way upward among the boulders. Pause to take in the wonders of the area one last time before departing the gorge floor and beginning the trip back to the rig.