Florida Keys, chain of coral and limestone islands and reefs, c.150 mi (240 km) long, extending from Virginia Key, S of Miami Beach, to Key West, and forming the southern extremity of Florida. Between the Keys and the mainland lies Florida Bay; they are separated from Cuba by the Straits of Florida. Many of the islands, the best known of which are Key Largo and Key West, have resort developments. The Keys are also noted for their commercial fisheries, diving zones, subtropical vegetation, and variety of wildlife. In the 1990s concern over population growth, increased traffic, and degradation of reefs and local water grew. The first U.S. undersea park, at Key Largo, has noted coral formations. Most of the islands are linked to the mainland by the Overseas Highway (completed 1938), which has 42 bridges.