The Lava River Cave is the longest uncollapsed lava tube in Oregon — about a mile from entrance to the back wall. It sits within the Newberry National Volcanic Monument, about 12 miles south of Bend on Highway 97. The cave formed roughly 80,000 years ago when a lava flow cooled on the outside and drained from the inside, leaving a hollow tube through the basalt.
Walking through it is a genuine experience. The passage is wide enough to stand in throughout most of its length, with the floor covered in broken basalt slabs and the ceiling showing the original flow marks. The temperature stays at 42°F regardless of the season — bring a jacket even in July. Lantern rentals are available at the entrance for a few dollars, which is the preferred way to explore, though headlamps work just as well.
The cave is open from late May through early September, and is managed by the Deschutes National Forest. It sits just off the highway and requires only a modest Northwest Forest Pass for entry.