“We have listened to the moan from three different points on this ridge,” Jupiter explained. “In my mind I drew imaginary lines from where I listened to where the moans appeared to originate. Where the three lines crossed is the exact source of the sound.”

Bob suddenly understood. “Sure, Pete,” he said. “It’s called triangulation. Engineers use it all the time.”

“Precisely,” Jupiter said. “Of course, the way I did it was very rough, but it will serve our purpose.”

“What purpose, Jupe?” asked Pete. “I mean, what did we find out?”

“We have found that the exact source of the sound is that cave in the mountain — El Diablo’s Cave,” Jupiter announced.

“Gee, Jupe,” Pete exclaimed, “we knew that already. Mr. and Mrs. Dalton told us.”

Jupiter shook his head. “Good investigators do not accept what other people report without checking it themselves. Witnesses are often unreliable, as Mr. Hitchcock has told us many times.”

Jupiter referred to the motion picture director, Alfred Hitchcock, who been a good friend of The Three Investigators ever since they had embarked on their adventures by trying to locate a haunted house for him to use in a film.

“I guess you’re right,” Pete said. “Mr. Hitchcock did show us how little witnesses really see.”

“Or hear,” Jupiter added, “But now I have no doubt that the moaning does come from El Diablo’s Cave. All we have to do is find out what is moaning, and —”

The stocky boy did not finish his sentence, for the moan rolled out once more — weird and chilling in the deep twilight of the shadowy valley. “Aaaaaa — hhhhhh — ooooooooooooo — oooooo — oo!”

Even Jupiter shivered this time, as long shadows began to creep across the valley.

Pete swallowed hard. “Gosh, Jupe, Mr. Dalton and the sheriff have searched the cave three times already. They didn’t find anything.”