Boulder Cave, on the Chinook Scenic Byway, is a rarity in the relatively cave-free terrain of the Pacific Northwest and doubly unique due to its formation through a combination of volcanic and erosive processes.
A 2-mile round-trip trail built by the Civilian Conservation Corps in 1935 leads into the cave’s murky interior, formed when Devil’s Creek cut a tunnel through soft sedimentary rock, leaving hard volcanic basalt on top. Up to 50 rare big-eared bats hibernate in the cave each winter, when it's closed to the public. Bring a flashlight.