A dying criminal confesses that his loot has been stored "in the tower." Both towers of the looted mansion are searched in vain. It remains for the Hardy boys to make an astonishing discovery that clears up the mystery and clears the name of a friend's father.

Frank and Joe Hardy are investigating a mysterious old house high on the cliffs above Barmet Bay when they are frightened off by a scream. The boys return to the apparently haunted house when they make a connection between the place and a smuggling case their father is working on. When their father goes missing, they have to investigate the caves beneath the house and confront the smugglers.

With two cases in tow, the Hardy boys look to Turner mill for clues. Determined to learn the secret of the old mill, Frank and Joe employ a clever ruse to gain entrance, only to find themselves trapped. How the young detectives extricate themselves from this dangerous situation and unravel mysteries will keep readers tense with suspense

Timber wolves, a Rocky Mountain blizzard, and a mine cave-in are only a few of the perils Frank and Joe Hardy encounter during their search for the principal members of a notorious gang responsible for a payroll robbery. Clue by clue, Frank and Joe cleverly fit into place the scattered pieces of this dangerous puzzle and come up with the astonishing solution.

With two mysteries to solve, Frank and Joe must climb aboard their iceboat the Sea Gull to reach Cabin Island, where a belligerent stranger orders them off. Sabotage to the boat, danger to themselves, and a ghostly prowler do not daunt the Hardy boys in their search for Johnny Jefferson and for clues to stolen antique medals. How the teenage investigators outwit a ruthless foe and succeed in solving both mysteries makes for mounting suspense in this brisk-paced adventure.

In their motorboat the Sleuth Frank and Joe Hardy search Barmet Bay for a dangerous stranger who has stolen a valuable boat. Suddenly, in the eerie fog, they spot the craft drifting aimlessly out to sea. What happens next starts the young detectives and their pal Chet Morton on an intriguing adventure that takes them to Mexico and into the comparatively unexplored desert and mountain regions of Baja California.

A series of museum thefts launch the Hardy Boys on this baffling mystery. Rare collections of ancient pirate treasure are being stolen, so Frank and Joe are asked to stake out the Black Parrot, a suspicious freighter docked in Bayport Harbor. This whirlwind chase after a self-styled pirate king in the Caribbean stronghold of the Empire of the Twisted Claw makes a gripping tale of suspense and high adventure.

A sleek new hydrofoil is scheduled to start ferrying passengers between Bayport and Cape Cutlass. But business enemies of the hydrofoil owner have stirred up a hornets' nest of violent opposition among small boat owners. Fearing sabotage, he begs Frank and Joe Hardy to guard the Flying Express on her maiden trip. Startling developments plunge the teenage detectives into a dangerous chase by sea, air, and land in pursuit of a gang of hardened criminals who operate by the signs of the Zodiac. Tension mounts when the Flying Express vanishes - and so does Sam Radley, Mr. Hardy's...

There is a crisis in Bayport Soon the city will be without water Every night, water strangely disappears from the new reservoir near Skull Mountain. Frank and Joe Hardy join forces with a team of skilled engineers to solve the baffling mystery.

An invitation from Cap Bailey, science teacher at Bayport High, to accompany him out West to Wildcat Swamp on an archaeological expedition triggers off a series of dangerous events for Frank and Joe Hardy. On their way West the boys and Cap have a near-fatal accident in a private plane which has been sabotaged. Though warned to leave the area, Frank, Joe, and Cap doggedly remain until they have caught the cunning ex-convicts they are up against in this swift-paced adventure.