The Secret of Pirates' Hill

The Secret of Pirates' Hill is Volume 36 in the original The Hardy Boys Mystery Stories published by Grosset & Dunlap.

This book was written for the Stratemeyer Syndicate by John Almquist in 1956. Between 1959 and 1973 the first 38 volumes of this series were systematically revised as part of a project directed by Harriet Adams, Edward Stratemeyer's daughter. The original version of this book was shortened in 1972 by Priscilla Baker-Carr resulting in two slightly different stories sharing the same title.

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.