Fort Jefferson and related facilities nearly cover Garden Key. The island, formed by shifting sands and coral boulders, has an area above water of about 16 acres.

After being abandoned for 24 years in the 1870's and 1880's the military returned in 1898 at the beginning of the Spanish-American War. During that war the island was used as a coaling facility. The coaling station station itself, the ruins of a portion of which are shown below, was constructed from about 1898-1904.