Plot summary

Every night water strangely disappears from the new Tarnack Reservoir near Skull Mountain. Frank and Joe join forces with a team of skilled engineers to solve the baffling mystery.

The book takes place on Skull Mountain, a mountain where many skulls have been seen, near Bayport, U.S.A. This city around Skull Mountain loses water each night because of the new reservoir. There is always something mysterious happening on the mountain, which has an underground channel.

The story begins when Joe wants to go swimming, however, Frank points out that there is no water. When they discover that the water is missing, they team up with Chet Morton, Dick Ames and Bob Carpenter in order to solve the mystery. While exploring Skull Mountain, where the reservoir is located, the boys are attacked several times. They finally find Timothy Kimball Jr. (sweeper) breaking into Mr. & Mrs. Kleng’s store in order to steal the safe which holds his pay ($5000). Timothy is arrested and questioned about the reservoir. This leads the Hardy Boys to catch the villain, Mr. Kleng, and solve the crime. At the end, the Hardy Boys can go swim because there is enough water in the pool.

Original edition (1948)

• Ghostwriter: George Waller Jr.
• Summary: The boys avert a crisis when they discover the reason why water is disappearing from a new reservoir in Bayport.

Revised edition (1966)

• Revision type: Minor revisions.
• Ghostwriter: David Grambs