That is the title that I think this book should be because there is another mystery going on at the same time that eventually involves the Three Investigators with the mystery of the moaning cave. The plot is trying to see how or what is making the cave moan only at night and not in the morning. I really liked this book because it kept me reading, even when I wasn't supposed to be reading the book.


Jupiter, Pete, and Bob solve the mystery of El Diablo   July 16, 1999
 3 out of 3 found this review helpful

The tenth (10th) installment in the "Three Investigators" series finds the boy sleuths travelling to a ranch to try to solve the mystery of 'Moaning Valley'. They uncover clues leading back to an 18th century bandit, and object of local folk legend named 'El Diablo'. The legend says El Diablo used the cave in Moaning Valley as his hideout, and disappeared into the cave 80 years ago... But suddenly there are stories that he's returned! The cave gives off an errie moan that terrifies anyone who hears it. The boys do a bit of scuba diving, and see an odd creature in the water on the western bank of the mountain where El Diablo's cave is located. The conclusion is both satisfying, and fun as they tie up all loose ends, and help the authorities solve a mystery at the same time. I re-read this story to my 6 year old son last month - still one of my favorites in the series.