Hired to locate an old Spanish cannon, the Hardy brothers uncover an even greater treasure after perilous underwater adventures.
While the Hardys were scuba diving out at Pirates Hill Frank air hose is punctured by a harpoon fo an unknown reason. Everytime they go to Pirates Hill they are warned and attacks are attempted on them.
Meanwhile the Bayport Historical Society has received cutlasses that are of vital importance to the finding of the cannon. They are eventually stolen by one their enemies that hope to find the cannon before the Hardys do.
This mystery had great suspense and the book didn't let on as to who was the real traitor was. The only thing I questioned was the plot because cannons aren't that big for criminals to go after. Other then that the book was great.
This is one of the best hardy boys books ever,and a must-have for any hardy boys fan!