Americans may be aware of what's above ground, but do they know what's concealed beneath? And could subterranean facilities on U.S. soil house secrets so dangerous, our government would stop at nothing to keep them hidden? We examine the tunnels, cities, and fortresses covertly constructed beneath the feet of the American public.

Cast: Jonathan Adams, Barry Aird, Mark Allin, Mark Allin, Gary Brierley, John Casti, Ryan Cragun, Robin Dornfest, Ryan Dube, Laura Duffy, Steve Duncan, Jeffrey Grossman, Jamie Henn, Terry Kirkpatrick, Joseph Lanier