Franklin W. Dixon

The Curse of the Ancient Emerald
Three nights. Three riddles. Three robberies.
 First, a priceless painting disappears from the Bayport Historical Museum; then, two ancient samurai swords vanish from a private collection. A string of robberies has hit Bayport, and all signs point to the Phantom, a recently released ex-con who last struck more than twenty years ago. There’s only one problem: The Phantom has vanished into thin air, and not even Frank and Joe Hardy can track him down.
 Then the boys begin receiving threatening notes in the mail, each one containing a strange riddle. And it looks like the Phantom’s next target could be the valuable Emerald of Astara, the 2,500-year-old crown jewel of the historical museum. Can the Hardys crack the Phantom’s code before he strikes again?
 
 
Tunnel of Secrets
Sinking into danger
 When a sinkhole opens up and swallows one of Bayport’s historic statues, Frank and Joe suspect something — or someone — shady is at play. They jump at the chance to investigate: Joe as a member of an urban exploration club and Frank as a photographer for the school paper.

 But in the thick of their search, the boys stumble upon an entire underground city that’s been abandoned for years. Now the city is home to a cultish band of criminals who plan to take over Bayport. Frank and Joe are determined to catch the crooks . . . but after being barreled down by a boulder and nearly squashed by a runaway train, they have to wonder: Are they in over their heads?