Grand Canyon Spill, Arizona, 2007

The canyon's rock strata, laid bare over the past six million years by the rushing Colorado River, details nearly two billion years of North America's geologic history. At 277 miles (446 kilometers) long, 18 miles (29 kilometers) across at its widest, and 6,000 feet (1,829 meters) down at its deepest, it is one of Earth's largest canyon systems.

(Text adapted from and photo shot on assignment for, but not published in, "The Unexpected Canyon," January 2006, National Geographic magazine)