What if the ghastly images and abominations haunting our collective nightmares actually exist? Writer/director Adam Green (Hatchet) sets out to make a documentary exploring this tantalizing premise after being contacted by a mysterious man named William Dekker (Ray Wise). Dekker claims he can prove that "monsters are real" and insists these grotesque creatures are forgotten, hideously deformed humanoids inhabiting a vast, underground metropolis of the damned. Determined to expose the truth, Green embarks on a bone-chilling odyssey and gets more than he bargains for when he dares to go Digging Up the Marrow.