About 12 miles south of Bend on U.S. 97 and 1 mile south of Lava Butte is Oregon’s longest known lava tube, the Lava River Cave (541/593-2421, 9 a.m.–5 p.m. daily July–Labor Day, 9 a.m.–5 p.m. Wed.–Sun. May–June and Labor Day–mid-Oct., admission with NW Forest Pass). The cave is a cool 40°F year-round, so dress warmly. Since the walking surface is uneven, flat shoes are recommended.

Bring a flashlight to guide you through this lava tube or rent a propane lantern ($4) at the entrance. The lanterns are recommended because they have no batteries to die out a mile deep in the darkness. The trail is an easy 2.4-mile round-trip from the parking lot.

The first chamber you enter is called the Collapsed Corridor. Volcanic rocks that fell from the roof and walls lie in jumbled piles. Freezing water in cracks pry a few rocks loose each winter, which is why the cave is usually closed during the cold months.

Stairs take you out of the Collapsed Corridor into a large void called Echo Hall. Here the ceiling reaches 58 feet and the cave is 50 feet wide. Conversations return from the opposite side of the hall as eerie noises in the dark. The lateral markings you see etched on the walls show the various levels of past volcanic flows.

At Low Bridge Lane, watch your head because the ceiling dips down to five feet. Here and in other areas of the cave look for the “lavacicles”—a term from a geologist’s 1923 publication on the cave called The Lava River Tunnel. Two kinds of lavacicles are found: The hollow cylindrical “soda straws” were formed by escaping gases, and the cone-shaped formations were created by remelted lava dripping down from the ceiling.

The next curiosity you’ll come across is a cave inside the cave, the Two Tube Tunnel. Two tubes intermittently connect for 95 feet. The smaller tube was formed when the level of the lava flow dropped and the cooling lava created a second roof and tube inside the existing cave.

The terrain changes again in the Sand Gardens. Rain and snowmelt carry volcanic ash down through cracks and openings in the cave and deposit them here. The process continues today with the nearly constant dripping water carving out spires and pinnacles in the sand. These formations take hundreds of years to grow, so stay out of the fenced-off area.

The sand gets thicker and thicker until it completely blocks off the lava tube, forcing an abrupt about-face. The walk back to the light of the sun affords a different perspective on this remarkable natural attraction.

It is important to avoid littering the cave, collecting samples, or doing anything else to mar this delicate ecosystem. Don’t light flares, paper, or cigarettes because the fumes kill off insects, a food source for the cave’s bat population. Roosting bats should not be disturbed because waking them from hibernation results in certain death for these winged mammals.

Incidentally, bat droppings support this cavernous ecosystem, and others; bat guano is harvested commercially and used for detoxifying waste, improving detergents, and producing antibiotics. Bats can catch hundreds of mosquitoes per hour, and they are also important pollinators. There are nearly 50 species of bats living in North America, and if left alone, they pose little threat to humans.

In short, “leave nothing but footprints, take nothing but pictures, and kill nothing but time.”

A couple of miles south of Lava River Cave is the Lava Cast Forest. Take U.S. 97 and go 9 miles on the cinder road (Forest Service Rd. 9720). From there, follow a self-guided trail through an unreal world created when lava enveloped the trees 6,000 years ago. The lava hardened, leaving behind a mold of the once-living trees, much like how the eruption of Mount Vesuvius in Italy left casts of Pompeii’s human residents.