In a series of hair-raising adventures both on land and undersea the teen-age brother detectives pit their wits against some of the most ruthless criminals they have ever encountered. It all starts when Frank and Joe are skin diving just for fun and the thrill of exploring the undersea world. Suddenly, deep in the waters that flow near the foot of Pirates' Hill, a mysterious skin diver fires a spear through Frank's air hose. From this moment on, danger is never far away. The very lives of the boys are at stake as they, with the help of their pals Chet Morton and Tony Prito, uncover a mystery involving an old Spanish cannon and a fabulous sunken treasure. Again, Franklin W. Dixon has woven a suspense-filled story that will thrill his many fans. Hired to locate an old Spanish cannon, the Hardy brothers uncover an even greater treasure after perilous underwater adventures.