Jerry held the nose of the Sleuth inshore and maneuvered so that the propeller remained in deep water.  He waited impatiently for news of Joe.

It did not take the others long to find that the big cave they had entered was unoccupied.  They reappeared a few minutes later.

"Did you find him?" Jerry called.

"No luck," Biff reported.

Chet was discouraged and said so.  "We're working on the slimmest of clues," he said.  "The fair-haired man and his friends might not have meant the Shore Road caves.  Don't forget, there are hundreds of subterranean caverns between Gresham and Bayport."

"But the caves here are the best known," Frank remarked.  "Let's look some more.  I'll cruise along the shore and pick out the more likely caves to hide a prisoner."

The motorboat edged its way along the face of the cliff.  Whenever the boys noticed one of the larger openings that could be reached easily from the shore, Frank ran the boat in among the rocks.  Then, while one boy stayed in the Sleuth, the others would scramble up to investigate the cave.

The hours dragged by.  Finally they navigated to a place where the cliff sloped and began to give way to sandy hills and wooded inclines.

Biff gave a sigh.  "Guess we'll have to give up.  There's only one small opening left to investigate."

"But why would kidnappers go way up to that cave when there are so many that are easier to reach?"  Chet protested.  "They'd have to climb fifty feet up to the mouth."

"It isn't so steep as it looks," Frank remarked thoughtfully.  "And I can see a sort of winding trail up the slope."

"I'm game," Jerry said.

"Me too," Biff added.

Frank brought the Sleuth in toward the rocks.  The boys craned their necks to look up at the tiny opening in the face of the cliff above.

"I guess you're right, Chet," Jerry admitted.  "Joe's kidnappers wouldn't climb all the way up there, with so many better caves to pick from."