The Ice Cave and Bandera Volcano are privately owned, not part of the National Park system. The $10 per person entrance fee seems rather steep to me, but hey!, we’re on vacation. The cost is offset by the savings from Dad’s senior citizen national parks pass, which gets us into many of our destinations for free.
Sometimes the outside of a lava river hardens while the inside remains molten and continues to run, leaving behind a tube. One of these lava tubes gathered water that then froze. Cold air settles to the bottom of the cave, and the porous lava insulates the ice. Conditions are right for the ice to stay in the cave permanently. The temperature remains at 31 degrees, and the amount of ice slowly grows over time.