What if the ghastly images and abominations haunting our collective nightmares actually exist? Filmmaker Adam Green receives a package in his fan mail from a mysterious man named William Dekker (Ray Wise, Twin Peaks, X-Men: First Class, Robocop) claiming that he can prove that monsters do indeed exist, insisting these grotesque creatures are forgotten, hideously deformed humanoids inhabiting a vast, underground metropolis of the damned. Determined to expose the truth, Green and his crew embark on a bone-chilling odyssey. As the lines between fantasy and reality are increasingly blurred, they are taken on a mysterious, fantastical, and terrifying journey into the shadows and deep down under the ground below our very feet when they dare to go Digging Up the Marrow.