El Malpais, New Mexico is a geologists dream. The area is literally littered with sharp, black lava rocks. If you hike across the rocks for any distance at all you will find that it cuts up the bottoms of your shoes rather like letting your dog chew on them.
You will also find that underneath the piles and mounds of rock are tunnels, tubes bored out by lava flowing across the ground. As the outside of the lava cooled it formed a crust and the inside flowed out of the crust, leaving a tunnel behind. Some of these lava tubes can be explored.
This one had a skylight, a round hole that had caved in some time before, leaving a gaping hole with trees looking down over the rim like it was their fault.