I've been looking for places to find bats in their natural habitat, rather than around the house. The author of [this question](https://outdoors.stackexchange.com/questions/17104/how-warm-does-it-need-to-be-for-bats-to-come-out-of-their-caves) saw some bats in a lava tube. I've never heard of a lava tube before, and would like to know what it means, and how it happens. 

I understand that lava flows from a volcano, and affects any type of terrain in its path, including where it lands on the ground, which can be quite a distance from the site of the eruption. 

How does lava create a tube-type formation with space inside? Is it hard enough to become something permanent, or is it a temporary structure that caves in at some point?

Is there any place in the United States where we could go to see one? I'm hoping we might find some bats inside.