Lava began spewing from deep cracks in the earth some 2,000 years ago. Rivers of molten, red hot rock crawled northward 16 miles, covering the floor of Hat Creek Valley.

While the top crust cooled and hardened, rivers of red-hot lava insulated by the newly formed rock above, continued to flow.

Eventually, the lava drained away, leaving tube-like caves. Subway Cave is the largest accessible tube in the flow....