Pre-Civil War Fort Jefferson, largest brick-masonry structure in the Americas, occupies much of Garden Key in the Dry Tortugas, and extends northward into the Gulf of Mexico over fill. As with many coastal forts, its walls are full of stories never to be told, and what we do know of its rich history only touches the surface. For example, its mid/late-1800s use as a military prison included the doctor who treated Lincoln’s assassin. On Images of the Week featuring forts, I’ve often used the completion year, but this one was started in 1846 (the year after Fort Zachary Taylor at Key West) and never fully finished. Only extensive WPA renovation in the 1930s, and subsequent patchwork, has kept it from much-worse deterioration. Regardless, even as parts of it have started to lose bricks and crumble to the sea, the fort has withstood many hurricanes and the corrosive effects of salt air and water remarkably well. Mineral deposits on the floor are similar to those in caves, but instead, dissolved from the grass-covered, limy sand that occupies the top level, and deposited through drips out of masonry cracks.