Prior to 1600, Spanish fleets returning to Spain from gold-laden Mexico sailed around the Gulf, hugging the shore. This early route brought the fleets precariously close to the Dry Tortugas. Many ships became casualties of the reefs and shoals surrounding the region. The earliest known shipwreck site in this area is from a 1622 Spanish plate fleet, although it is reasonable to assume that there are many earlier and undocumented wrecks within the park. Sinkings and strandings continue to occur here.
Material remains of war, commercial disasters, and private wrecks dot the underwater landscape.
Many of these have yet to be fully discovered, and some will forever remain hidden on the bottom of the ocean under vast amounts of sand. When the United States acquired Florida from Spain in 1819, Garden Key became a prime location for military fortification to protect the shipping lanes between the Mississippi River and Europe. In 1825, a lighthouse was constructed on Garden Key to warn sailors of the rocky shoals. Construction of an immense fort began in 1846, but was never completed, due to a number of circumstances. Settling caused cracking in the walls and in the 100-plus cisterns, used to store 1.5 million gallons of rainwater for use on the island.