In 1847, the U.S. military built Fort Carroll to protect Baltimore right in the middle of the Patapsco River. The site was selected because experience showed that a defensive fort built too close to a city created more problems than it solved. The artificial island was built under the supervision of a young Robert E. Lee, who also designed the island’s hexagonal shape.
The fort was still incomplete by the time the Civil War began. Construction was halted, and by the time the war was over, the facility’s insufficiency became obvious. The fort was modernized, but not in time to be of much use during the Spanish-American War.
Every time the fort was slowly modernized, it became obsolete again. By 1921, the army had abandoned Fort Carroll for good. The island was sold to a private developer in 1958, but various plans to use it proved too difficult and expensive to carry out. The fort remains, though slowly crumbling into ruin.