|Franklin W. Dixon|
|Pages/Publication Date:||192 / 2016|
|Publisher:||Grosset & Dunlap|
"Drop case or else danger for you and your family." This written warning addressed to Fenton Hardy not only alarms the famous investigator and his detective sons, but also puzzles them, because Mr. Hardy is working on more than one case. When more ominous warnings follow, Frank and Joe suspect there is a link between the counterfeiting case they are investigating and a secret case their father cannot discuss since it involves national security. In this third book in the series, the key to the solution of both cases appears to be hidden in the old Turner mill; the brothers can get in, but once they do, they are trapped in a subterranean world.