Estimated to have been formed roughly 10,000 years ago by Newberry Volcano, Boyd Cave is an 1,880-foot-long lava tube located roughly 12 miles southeast of Bend, Oregon. Well preserved and featuring pāhoehoe and ʻaʻā basalt formations, Boyd Cave can only be accessed by a small 6-foot-diameter oculus in the cave's ceiling. By venturing down 20 feet via a set of sturdy steel stairs the visitor will quickly enter into a unique and chilly underground setting.
Lava tubes form as the top layer of a lava flow is exposed to air. The air cools the lava, which slows as it begins to solidify. Meanwhile, the lava lower down remains at temperatures as high as 2,000 degrees and continues to flow like a river below the hardened top crust. If the river has the opportunity to drain, the lava tube remains.
According to Wikipedia:
Note: Be sure to dress warmly as temperatures in the cave remain around 42 degrees year round, and be sure to bring headlights, flashlights, or lanterns for visibility and safety. While you are in the area, be sure to visit nearby Skeleton Cave (by special permit only, call 541.389.8359 for details), Arnold Ice Cave, Hidden Forest Cave or Pictograph Cave.