Sinking into dangerWhen 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?