When Frank and Joe set out to explore caves and tunnels, they dig themselves into some dark and dangerous trouble in this Hardy Boys adventure.<br><br>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.<br><br>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 shut down 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?