Overseas Railway
The Keys were long accessible only by water. This changed with the completion Henry Flagler's Overseas Railway in the early 1910s. Flagler, a major developer of Florida's Atlantic coast, extended his Florida East Coast Railway down to Key West with an ambitious series of over-sea railroad trestles.

Labor Day Hurricane of 1935
One of the worst hurricanes to strike the U.S. made landfall near Islamorada in the Upper Keys on Labor Day, Monday Sept. 2. Winds were estimated to have gusted to 200 mph, raising a storm surge more than 17 feet above sea level that washed over the islands. More than 400 people were killed, though some estimates say that more than 600 died in the terrible storm.

The Labor Day Hurricane is one of only three hurricanes to make landfall at Category 5 strength on the U.S. Coast since reliable weather records began (about 1850). The other storms were (Camille (1969) and Andrew (1992).

In 1935, new bridges were under construction to connect a highway through the entire Keys. Hundreds of World War I veterans working on the roadway as part of a government relief program were housed in unreinforced buildings in three construction camps in the Upper Keys. When the evacuation train failed to reach the camps before the storm, more than 200 veterans perished. Their deaths caused anger and charges of mismanagement that led to a congressional investigation.

The storm also ended the 23-year run of the Overseas Railway; the damaged tracks were never rebuilt, and the Overseas Highway (U.S. Highway 1) replaced the railroad as the main transportation route from Miami to Key West.

Seven Mile Bridge
One of the longest bridges in existence when it was built, the Seven Mile Bridge connects Vaca Key (island town of Marathon, Florida) in the Middle Keys to Bahia Honda (pronounced ba-EE-uh OWN-dah in Spanish) in the Lower Keys. True to its name, it is seven miles or about 11km long, and passes Pigeon Key, where section of the old bridge allows access to the small island.

After the destruction of the railway by the hurricane, it and the other bridges were rebuilt by the United States Federal Government as an automobile highway. US 1 runs the length of the Keys, and is called the Overseas Highway there. (US 1 also runs the entire way up the eastern seaboard to Maine.)

Conch Republic
In 1982, the United States Border Patrol had established a roadblock and inspection points on US Highway 1, stopping all northbound traffic returning to the mainland at Florida City, to search vehicles for illegal drugs and illegal immigrants. The Key West City Council repeatedly complained about the roadblocks, which were a major inconvenience for people traveling from Key West, and hurt the Keys' important tourism industry.

After various unsuccessful complaints and attempts to get a legal injunction against the blockade failed in federal court in Miami, on 23 April 1982 Key West mayor Dennis Wardlow and the city council declared the independence of the Keys, calling it the "Conch Republic". After one minute of secession, he (as "President") surrendered to an officer of the Key West Naval Air Station (NAS), and requested one billion ($1,000,000,000) dollars in "foreign aid".

The stunt succeeded in generating great publicity for the Keys' plight, and the inspection station roadblock was removed.