Fort Jefferson's northwest bastion, looking out over the North Coaling Dock Ruins. One of the reasons the United States would have wanted a starfort way out here in the middle of noplace was...coaling! The steamships of the 19th century could only steam as long as their coal lasted, so it behooved any nautical-minded nation to have lots of coaling stops scattered around the world's oceans. The sign we see precipitously close to the edge is Defending US Commerce.