Key Largo is an island in the upper Florida Keys archipelago and, at 33 miles (53 km) long, the largest of the Keys. It is also the northernmost of the Florida Keys in Monroe County, and the northernmost of the Keys connected by U.S. Highway 1 (the Overseas Highway). Its earlier Spanish name was Cayo Largo, meaning Long Key.
Key Largo is connected to the mainland in Miami-Dade County by two routes. The Overseas Highway, which is U.S. Highway 1, enters Key Largo at Jewfish Creek near the middle of the island and turns southwest. Card Sound Road connects to the northern part of Key Largo at Card Sound Bridge and runs southeastward to connect with County Road 905, which runs southwest and joins U.S. 1 at about mile marker 106. These routes originate at Florida City on the mainland.
Key Largo is a popular tourist destination, and calls itself the "Diving Capital of the World" because the living coral reef a few miles offshore attracts thousands of scuba divers and sport-fishing enthusiasts.
Key Largo's proximity to the Everglades also makes it a premier destination for kayakers and ecotourists. Automotive and highway pioneer and Miami Beach developer Carl G. Fisher built Key Largo's famous Caribbean Club in 1938 as his last project.
The island gained fame as the setting for the 1948 film Key Largo, but apart from background filming used for establishing shots, the film was shot on a Warner Brothers sound stage in Hollywood. After the film's success, pressure from local businesses resulted in a change in the name of the post office serving the northern part of the island, from Rock Harbor to Key Largo, on 1 June 1952. After that, every resident north of Tavernier had a Key Largo address and the cancellation read Key Largo.
Three census-designated places are on the island of Key Largo: North Key Largo, near the Card Sound Bridge, Key Largo, eight or nine miles from the southern end of the island, and Tavernier, at the southern end of the island. Ocean Reef Club is a private gated community and club at the northern end of the island. None of Key Largo is an incorporated municipality, so it is governed at the local level by Monroe County.
Key Largo is situated between Everglades National Park to the northwest and John Pennekamp Coral Reef State Park to the east, the first underwater park in the United States and the site of the only living coral barrier reef in the continental United States.
Geology and geography 
The island of Key Largo is an exposed, fossilized remnant of a coral reef formed during a period of higher sea level and then uncovered and eroded during a subsequent ice age. The highest elevation is a slight ridge forming the spine of the island, which rises to about 15 feet (4.6 m).
The island's substrate is called Key Largo limestone; in many places, fossilized corals and smooth, eroded limestone "caprock" are visible at the surface. Solution holes, which are pockets dissolved in the limestone by acidic rainwater, form shallow depressions in the land. The natural shoreline of the island is generally rocky. A slippery, gray, limestone-based clay called "marl" is the shoreline and near-shore soil. There are no natural sand beaches on the island. Inland, decomposed vegetation forms a rich, acidic humus soil up to about six inches thick, topped by "leaf litter". The soil supports a diverse flora of herbaceous plants, woody shrubs and hardwood trees.
Key Largo's climate is tropical (Aw or tropical savanna in Koppen Climate classification). Frost has never been recorded in the island. Key Largo enjoys close to 3000 hrs of sunshine annually.
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