USN soldiers place naval mines at the entry near Fort Pickens, Florida during World War I. 

These are U.S. manufactured Mark 6 naval mines holding 300 pounds of explosives to cause damage to German submarines trying to patrol close in to Florida shores. This crew were onboard the US minelaying vessel USAMP General Royal T. Frank*. Mine laying vessels like this one were assigned to the US Coastal Artillery Corps, Mine Planter Service under US Army command. Fort Pickens assisted in naval mine warfare as the installation had several mine support facilities for storage and deployment into the Gulf of Mexico.
This vessel would serve through the inter-war years and would later be sunk during World War II by the Imperial Japanese Navy submarine IJN I-71 30 miles North of Upolu Point, Alenuihaha Channel, Hawaii 28 January 1942.

Source: US Army Signal Corps Historical Files.
National Park Service, Florida Coastline.
World War I. Off the Coast of Fort Pickens. Pensacola, Florida. 1918.
*Named in honor of Brig. Gen. Royal Thaxter Frank, Civil War veteran and commandant of the Artillery School 1888–1898