Less than 20,000 yeas ago, the lava of the Hat Creek flow was discharged in large volumes from a series of north-south fissures (cracks in the earth). This river of lava located near the town of Old Station, 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 newly formed rock above, continued to flow. Eventually, the lava drained away, leaving tube-like caves. The entrance to the cave was formed by a partial collapse of the cave's roof many years ago. Subway Cave is the largest accessible tube in the flow.