I’m a bit of a Google Maps/Google Earth nut and like just randomly browsing over countries and area’s I’m unfamiliar with (and will maybe never be able to afford to actually visit)
One day I was looking at satellite images off the Florida Keys (as you do) and came across a weird structure, clearly artificial in an area called Dry Tortugas. I’d found Fort Jefferson:
Below: Fort Jefferson, within Dry Tortugas National Park, Florida Keys
Fort Jefferson is a long-abandoned former military facility, essentially a fortress. It was originally built with the goal of suppressing piracy in the Caribbean and protect US shipping, but was also ultimately manned during the US Civil War.
Fort Jefferson was at one point heavily protected – muzzle-loading cannons were brought to bear on encroaching enemies, whilst the facility itself was surrounded by a moat, a throwback to castles of the Middle Ages.