After the United States' entry into the Spanish American War, Tampa citizens demanded increased coastal defenses, and in November 1898 construction began on a new military post on Mullet Key. Fort De Soto, named in 1900, included 29 post buildings such as barracks, a hospital, stable, guardhouse, administration office, mess hall and kitchen, bake house, storehouse, and workshops. Two mortar batteries defended the fort: Battery Laidley, the primary battery, housed eight 12-inch M 1890-MI mortars with a range of 1.25 to 6.8 miles, while Battery Bigelow held two 3-inch, 15-pound Driggs-Seabury Model 1898 rapid-fire guns needed to stop smaller, faster vessels and protect the 1.25-mile area surrounding the fort that Battery Laidley could not. Four mortars from Battery Laidley remain and are the only mortars of their kind in North America, with the only others in the world in the Philippines. Battery Bigelow collapsed into the Gulf of Mexico, but its ruins can still be seen in the surf southwest of Battery Laidley. The Fort De Soto Batteries and Military Post was listed on the National Register of Historical Places in 1977 and designated a local historic landmark by Pinellas County in 2014.