In Western Australia, visitors can tour two unusually accessible coral reefs. The reefs look similar enough to be fraternal twins, but they are separated in time by 400 million years. Ningaloo is a modern reef where visitors can snorkel amid spectacular reef-building organisms just a few meters from shore. It boasts one of the planet's healthiest reef environments, where 500 species of fish, 600 species of shellfish and more than 250 species of coral thrive. Ningaloo's twin, the now-dry Devonian-aged "Great Barrier Reef," is widely recognized as the world's best example of an ancient barrier reef, with creamy limestone layers providing an unparalleled window into the past.