The area of the inland western desert was once volcanic. Massive lava beds can be found from Southern California and Arizona all the way to Oregon and Washington, and these often contain caves made up of lava tubes. Lava tubes form when lava tries to flow out of fissures in a smooth, river-like mass called a Pahoehoe. There are several types of lava tubes. most of which are described in the book. A lava cave system made of multi-level tubes can be quite large and complex. This interesting geological feature is the setting of most of the book.
The caves that hide the remaining “wild” humans were discovered by Jeb when he was a young man working on a ranch. He fell through the roof of the large, surface cave and wandered for days before finding his way out. The caves have dark, purpley brown walls and ceilings. The walls are riddled with shallow holes that Wanda refers to as Swiss cheese.
The lighted upper caves would have been surface tubes which form only a few feet below the surface of the land from massive air bubbles that form and harden near the surface as the lava cools. The ceilings can be thin and brittle, which would explain how Jeb fell through the ceiling when he first found the caves as a young man. These surface tubes also form the large rooms that were described in The Host. Fissures and minor tubes often lead away from the large surface chambers, connecting them to other chambers or lower tubes and rooms. (Source: http://en.wikipedia.org/wiki/Lave_Cave )
According to Jeb, the caves were formed by an almost dead volcano, the only remnant of which is the hot water vent that still flows through the tubes. The water in the hot stream is heavy in minerals and sulfur. The river comes from deep in the earth and is heated by the once active volcano. The residents call it a hot spring. Besides the hot river, there is a clean underground stream that is part of either an underground spring or river system. Over the millennia, water carved and eroded the caves, openning them up and making the surface of the rock softer. The lower caves have stalactites and stalagmites showing they still have active hydrology either from the stream or from rain run-off.
The garden room, large dining room, and several passages are close to the surface and have holes that allow light in. Jeb openned up several of these to allow more light in the caves. The residents use a series of well placed mirrors to enhance the natural light and illuminate the caves during the day.