Lava River Cave

The Lava River Cave near Bend, Oregon, is part of the Newberry National Volcanic Monument, which is managed by the United States Forest Service. At 5,211 feet in length, the northwest section of the cave is the longest continuous lava tube in Oregon. While the cave's discovery in 1889 was officially credited to a pioneer hunter, the presence of obsidian flakes near the cave has led archaeologists to conclude that Native Americans knew about the cave long before settlers arrived in central Oregon.

Source: Wikipedia