Plot summary

Hired by a mysterious businessman to locate an old Spanish cannon, the Hardy brothers and friends grow more and more suspicious as they encounter stolen cars and a mysterious man on a motorcycle. They eventually uncover the cannon and thousands in gold bullion after perilous underwater adventures.

Original edition (1956)

• Ghostwriter: John Almquist
• Summary: Hired to locate an old Spanish cannon, the boys encounter dangerous criminals as they dive for the sunken treasure.

Revised edition (1972)

• Revision type: Minor revisions.
• Ghostwriter: Priscilla Baker-Carr