A lava tube is formed when the crust hardens but the lava below continues to flow. They are actually pretty common. There was one in the Park that was "groomed" with a walkway and lights that was big enough to drive a bus through. Most are smaller and dangerous... there is always the risk of collapse, and many have poisonous gases that require advanced breathing gear.
Welcome to all. I began blogging Retained-Images in 2004 at My-Expressions.com. It is gone now.