When you see a road sign that reads “Caution, Falling Rocks,” what size rocks do you expect to see? Pebbles? Softballs? Bowling balls? Five-story buildings? If you answered the last, you must live in Arizona where a 50-foot tall, 30-foot wide, 500,000-pound slab is breaking away from a canyon wall and threatening to fall onto the power and water facilities for the Glen Canyon Dam. Oh, and on a road too. I wonder what that sign says. What’s worse, the rest of Arizona is breaking apart too and there are many reasons why.

This is just the latest mysterious crack in Arizona. In March 2013, a fissure in Luepp became so big that it had to be fenced off. At the time the crack measured 900 feet long and 500 feet deep. The US Geological Service had no explanation for the crack and no predictions on how big or deep it could get.