This was an amusing feature at a campground near Taylor River Reservoir in Colorado: Campers had taken to setting up stacked rocks all over the grounds. I found more than a dozen of these things scattered around, and I did rather a lot of walking around all of the campground loops to capture as many examples as I could find. While historically these rock stacks would be used to mark important places or to warn people about things, the ones at this campground were done just for fun.
Yes, I was surprised by that, too. My dad and I tried making some stacks of our own, and we must have been fiddling around for half an hour before we got them to stop toppling over.