A coastal fort is built to protect something. The guns set at the mouth of Tampa Bay were not placed there to guard empty water. They were placed to guard a channel that, by the turn of the twentieth century, carried one of the most valuable cargoes in Florida out to the world: phosphate rock, the raw material of fertilizer. The story of why Fort De Soto exists at all runs straight through the story of that trade.
The discovery was an accident of a federal survey. In the winter of 1881, Captain J. Francis LeBaron, a civil engineer with the U.S. Army Corps of Engineers, was surveying the Peace River, southeast of Tampa Bay, for a proposed canal. He noticed odd pebbles in the riverbed and recognized them as phosphate, the mineral that makes fertilizer and, until then, something American farmers had to import or grind from bone. The find sat quiet for a few years. Then, in 1883, hard-rock phosphate mining began near Hawthorne, and in 1888 the richer pebble deposits of the inland district were opened. What followed was one of Florida's first great industrial rushes.
Within a decade of LeBaron's discovery, more than two hundred companies were mining phosphate in the counties east and southeast of Tampa Bay, and the price of an acre of Peace River land had leaped from a dollar and a quarter to roughly three hundred. The district became known as Bone Valley, for the spectacular Ice Age fossils, the bones of mastodon relatives, giant sloths, and sharks, that came up with the ore. The land-pebble beds there, buried under only a shallow overburden, were cheaper to work than river dredging or north Florida's hard rock, and Bone Valley became, and remains, one of the great phosphate districts on earth, the fertilizer of farms around the world.
All of that rock needed a way to the sea, and the sea-gate was Tampa Bay. The man who built the connection was Henry B. Plant, whose Plant System of railroads and steamships stitched the interior to the coast through the 1880s and 1890s. Plant developed Port Tampa, a deep-water transshipment hub roughly nine miles below the city, where his rail cars met his ships. By the late 1880s Tampa had become a major commercial port, sending out cigars and, increasingly, phosphate in enormous volume. Every ton of it, and every ship that carried it, passed through the single deep channel at the mouth of the bay, between Egmont Key and Mullet Key.
This is the link that is usually left out of the fort's story. A channel that carries a fortune is a channel worth attacking, and worth defending. When the United States rebuilt its coastal defenses under the Endicott program at the close of the century, Tampa Bay made the list precisely because of what flowed through it. The batteries at Fort De Soto on Mullet Key and Fort Dade on Egmont Key were built from 1898 onward to deny that channel to a hostile fleet, to keep an enemy cruiser from steaming in to shell the port or throttle its trade. The mortars and guns were aimed, in the end, at protecting the phosphate road to the sea. The fort guards the money.
The trade never left. Phosphate still moves through Tampa Bay in great quantity, and its modern legacy is written on the landscape and the water alike, in the flat-topped gypsum waste mountains east of the bay and in pollution episodes such as the Piney Point releases. The same channel that the fort was built to guard remains one of the busiest and most consequential waterways in Florida, and the cargo that first made it worth defending is still passing the old gun batteries on its way to the Gulf.