The rising action includes meeting the hermit, being shot at, finding the submarine, and exploring the caves.
The Hardy Boys: The Secret of the Caves was the seventh book in the Hardy Boys series. You will remember that we discuss plot in literature in terms of five consecutive parts: exposition (introduces the characters and conflict), rising action (events develop along the conflict), climax (the turning point), falling action (resolving conflicts that remain) and resolution (the ending, conflicts resolved).
To the find the rising action, the first step is to find the climax. The rising action is the events between the beginning and the climax, so you have to find the climax first. The climax is the most exciting part, so it’s usually easy to find. It is the turning point of the story. In a mystery, it is the point where the characters solve the mystery.
The initial conflicts in this story are that Morgan Todd is missing and the Coastal Radar Station is being sabotaged. The climax is when the boys discover the underground passage and the sabotage equipment. By way of resolution, they alert the Navy to rescue Morgan Todd and arrest the culprits who have been holding him hostage.
That leads us back to the rising action. This is the part where the boys meet the hermit and discover the submarine.