The picture shows an abandoned mine that has been covered with a grating to keep people from falling in. That box on top is about 15 feet from the (visible) bottom and the sides are vertical, so a fall would most likely be fatal. I saw a couple of rattlers down there the first time I was out there, too. A narrow opening at the visible bottom goes deeper, but I have no idea how much deeper. The box is designed to allow flying animals to get in and out. I hiked out there just before sunset recently and took along my bat detector. At first, I thought I was picking up some interesting stuff with the bat detector.
It was like something in there was turning on and off.
A little later, I noticed there was something suspicious about the way the detector was going on and off. I had noticed before that the detector is very sensitive to EMI. For example, it goes nuts if you hold it close to a CFL.
Demonstration of interference.
Anyway, with all the electronics I was carrying, I'm not sure how much of the things the detector detected were other stuff or just me and all my gadgets. I finally saw two bats fly out of the hole. That was disappointing. But, I also saw three owls come out. That was really cool. It was much too dark to get photos by then. One of the owls kept circling over me and screeching. It sounded kind of like a zone tailed hawk. I guess it didn't like me, so I left. It didn't stop screeching until I was about 200 yards away from the mine.
The reason I was hiking in that general area in the first place is because of something I noticed on Google Earth. There are saw-tooth like lines in the desert along Bush Highway there. I went out there to see what they were. They are channels cut into the ground to direct water so that it only crosses the road in a few places.