During production James Kenneth Davenport actually played the role of "Charles" the monster's hand in one scene.
During production the oxygen levels in the tunnel dropped so low at times to 18% that the crew and actors started to become paranoid and see and hear things that didn't exist.
The tunnel location has apparently been investigated by a paranormal investigation society who picked up "interesting" readings. They picked up an underground mist on photos that wasn't always visible to the naked eye. A chilling phenomena that the crew encountered also.