The Hardy boys' search for stolen gold leads them to an abandoned mine where, in the depths of the earth, they are attacked by outlaws.

Fenton Hardy, the famous private detective and father of the Hardy Boys, asks his sons to help him with his latest case involving a criminal named Snattman and the smuggling of stolen drugs.

When the Hardy boys eagerly agree to assist their friend Jack Dodd and his father in locating a family treasure, the two young detectives are unaware of the baffling complications that will arise. The only clue to the long-lost treasure is a cryptic message written by a Pilgrim ancestor. But when Mr. Dodd and Jack are accused of stealing cars and then mysteriously disappear, Frank and Joe are confronted with a triple mystery. Have their friends run...

While searching for the guard end of a broken saber that will solve one mystery, the Hardy brothers become involved with a gang of bank robbers.

Relates seven cases in which the Hardy brothers use police technology to track down criminals. Details of these techniques and procedures are given in the last five chapters.

The Hardy brothers' visiting Greek pen pal enlists their help in searching for a priceless ancient Greek helmet loaned to and lost by a Hollywood movie company years before.

Frank and Joe try to prevent the theft of a valuable collection of diamonds from a show.

The Hardy boys embark on another puzzling mystery when they try to discover who is stealing rare collections of ancient pirate treasure and smuggling them out of the United States.

Hired to locate an old Spanish cannon, the Hardy brothers uncover an even greater treasure after perilous underwater adventures.

While searching in Iceland for a missing sailor sought by an insurance company, the teen-age detective brothers uncover a plot threatening the life of a United States astronaut and the secrets of NASA's moon project.