In the late 1980s on the north side of South Mountain in Phoenix, Arizona, there were loads of entrances to mining tunnels that had not been closed when the mining stopped decades ago. My young friends and I were not so careful at that time, but we were fascinated by these mysterious realms. Some of the tunnels you can walk into while others do you have a slide down the gravel to get into, the latter of which made it difficult to get out of but it wasn’t impossible. Course, there was absolutely no light, so we had to bring flashlights with us. In some tunnels we could only walk 20 minutes before finding the end. In other tunnels, you could walk (slowly) for an hour and not find the end. The tunnels were all rock. Inexplicably, in some tunnels, there were meter wide “slots” at 45 degree angles from the foot path. A flashlight couldn’t[t penetrate all the way down into them, The worry was if you slipped into one, you had no idea how far down or where it went to. Read later that the state closed up some of the tunnels, but I haven’t been back in years to see.