Guadalupe Mountains

Photograph by Jonathan Blair/Corbis

The Guadalupe Mountains in Texas are home to one of the world's largest exposed fossil reefs, a 400-mile-long (644-kilometer-long) horseshoe-shaped limestone bed laid down some 250 million years ago during the Permian period. At that time, much of what is now Texas and New Mexico was covered by a vast tropical ocean. As this sea began to evaporate, sediments entombed the reef, perfectly preserving thousands of marine fossils.