Extensive public lava tube north of Mount Shasta (Klamath National Forest): roughly 1.5 to 2 miles of passages between collapses, ~190,000 years old, with about the first 1,200 ft safely walkable without a permit. Bring a strong light and a helmet; the far system runs on in total darkness.
California
See it on the map
Other places on the map within a couple of hours.
