When fluid, molten lava flows out of the ground, it works its way downhill. Soon the surface of this lava stream cools and hardens into a crust. Although the outer crust is hard, the lava inside is still molten, and continues to flow downhill. Once the molten lava has passed through, it leaves an empty tunnel called a lava cave, or more commonly, a lava tube.
The following is a partial list of National Park Service units that include lava caves: