This is an affordable campground within the grounds of Caverns of Sonora.  Someone suggested this as a must see and it was very unique.  The campground is basically a parking lot and we were one of two campers.  They don't take reservations and it was raining and cold.  The staff is amazing - it's always great to meet interesting people who care about their work and love sharing their unique property.

There is a cavern tour which takes you down into the dark cavern.  If you have claustrophobia, don't go.  We were okay but you walk through darkness the the guide turns on lights which illuminate these unique caverns.  It's pretty cool but I started to wonder if the air had a lot of CO2 because my husband and I felt slightly queasy - similar to altitude sickness.  I can imagine it's hard to get air way down under the earth.  

The town in Sonora is small and has a few places to buy groceries but it is really pitch dark at night and we took a wrong turn and found ourselves on a dark road.  If you like geology and/or caves, it's great to visit and stay.