Sipapu Bridge in Natural Bridges National Monument, Utah

Bridges to the past

The story of this striking sandstone formation in the US state of Utah begins around 10 million years ago. That's when tectonic shifts began slowly lifting the 336,000 square-kilometre Colorado Plateau above the surrounding plains. Later the Colorado River's many streams cut through the elevated land, threading it with deep canyons. When water broke through one canyon wall into another canyon, sometimes a natural bridge like this one remained above the gap.

The Natural Bridges National Monument - a protected area similar to a national park - protects three major bridges: Sipapu (pictured, and the largest), Kachina, and Owachomo. But its status can't stop the slow ravages of time. Thousands of tonnes fell from the inside of Kachina in 1992, slimming the bridge, and the remains of many fallen bridges dot the monument, hinting at the eventual fate of those that remain.