Coral is formed by colonies of tiny marine organisms called polyps that extract minerals from seawater to create hard carbonate exoskeletons. Accumulations of various types of living and dead coral are what make up coral reefs. Coral reefs form in warm, clear, sunny, agitated waters like those found along coastlines in tropical regions. The Florida Keys are the remains of coral reefs formed about 150,000 years ago and later exposed when sea levels dropped. The 190-mile-long Florida Reef Tract lying about 6 miles to the southeast of the Keys today is the only living coral barrier reef in the continental United States and the third-largest barrier reef in the world. Coral reef environments are incredibly rich in bio-diversity, a paradox since they mainly grow in nutrient-poor waters. In the shallow waters landward of the main reef along the Keys, numerous areas of coral growths known as patch reefs also provide important habitat for many species.