The Florida Keys archipelago, comprised of some 800 islands estimated to be up to 20-30 million years old, are made up of limestone rock covered by ancient coral reefs that became exposed over many years. Parts of the Keys consist of
Miami limestone, a Pleistocene marine limestone made up of fossilized creatures. A healthy, living reef stretches along the Keys today offering natural and artificial reef structures for divers to explore. The reef is the only living coral reef in the continental United States.
Early Florida Keys inhabitants were the Calusa and Tequesta Native Americans. Later, these tribes either left or were taken over by the more aggressive Seminoles. Spanish explorers first visited the Keys in the 16th Century. The origin of the Keys' name is still undetermined, but one theory is that they were named for the Spanish word "cayos" meaning small islands.
From there the Keys changed hands a few times from Spain, to Britain, back to Spain again and finally in 1812 the Keys became an American territory. The first settlement was in Key West in 1882 and for years, travel from key to key was only possible by boat. It was the dream of one Florida Keys entrepreneur, Henry Flager, to connect the Keys by rail. He finished the building of the Keys extension to the Florida East Coast Railway in 1910 and from that time until the hurricane of 1935, the Keys and especially Key West became a popular place to visit.
When the hurricane destroyed much of the Keys and the railroad in 1935, rebuilding started and as a result, the Overseas Highway was constructed to connect the islands by bridge. The highway was completed in 1938 and was known as the longest over-water highway. Today there are more than 40 bridges throughout the Keys and the longest is the 7-Mile Bridge in Marathon.