On a recent road trip, we stopped by Tonto Natural Bridge. It's hidden deep in a canyon in central Arizona. That's it on the right. It doesn't look too impressive until you realize that those little dots deep inside the arch are people. This shot was taken from an overlook maybe 200 feet above the creek. The travertine wall formed as a constant spring deposits calcium carbonate onto the trees and roots that grow there, encasing them in rock. It grew until it formed a dam in the creek, but the constant wear of the creek eventually wore a hole through the wall and created the bridge. It's still growing. The little spring continues to deposit minerals and drip over the bridge, while down below, Pine Creek keeps flowing. Calcium carbonate is the same stuff that makes white spots on your bathroom fixtures and plugs your pipes, but here, it's creating a miracle. And it's not the only natural wonder in the canyon. The spring dripping down a canyon wall has...