When a sinkhole opens up and swallows one of Bayport's historic statues, Frank and Joe suspect something--or someone--shady at play. They jump at the chance to investigate ; Joe as a member of an urban exploration club, and Frank as 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...