Adventure, danger, action and mystery all under one cover. This may be written for the juvenile market but it has all the elements of an exciting read.
Pat lives with his Grandfather on the island of Barinish, just off the coast of Ireland. The coast is rocky and harsh, but the sea is the source of much of the island's economy. It is set in a time before modern conveniences, so people know each other pretty well and interact on a regular basis.
When Pat decides to explore the Singing Cave he gets his Grandfather to come along. Always up for an adventure, Pat's grandfather pilots the light boat to the mouth of the cave. What Pat finds when he enters astounds him. After telling his grandfather about the find they decide to share the information with Mr. Allen—the most learned man they know.
From there things take a strange turn and the adventure and mystery begin. Rough seas, stow aways, late night movements, treasure missing and more.
It isn't a long read (192 pages) but it is a good read and fun to take your time with.
Another Goodread.