Without doubt, UAC's most remarkable recent discovery was uncovered in its expedition to Hell's Kadingir Sanctum. After several kilometers through nearly impassable terrain, the team discovered a sealed tomb - its entrance and walls covered in protective runes and imprisonment incantations. Once opened, the tomb revealed many artifacts including the Praetor suit. Most notable, however, was an enormous sarcophagus bound to the center bedrock of the tomb with thick iron bands - seemingly anchored to prevent anything from getting in or out.
It was initially believed this sarcophagus must hold a rare or powerful demon, but when later opened, it revealed the body of a man. The body was not petrified or decomposed - in fact, he appeared to be only sleeping despite the fact that the bed he lay in seemed millennia old. Attempts to wake the man were fruitless - and to harm him even more so - as a protective Argent barrier around the body kept him safe from harm in permanent stasis.
UAC archivists cataloged the discovery DM1-5, but project personnel had soon dubbed him the "DOOM Marine". While the history of this man remains conjecture, the Helix Stone, as well as other artifacts found at the Argent Fracture and during the Hell Expedition of ECm13, have shed some light on his identity. An etching in the book Daeva (another discovery of the Kadingir expedition), depicts the DOOM Marine wearing the Praetor suit, engaging demons in battle as a hooded figure looks on. This image had been previously observed numerous times in other artifacts, but only with the actual discovery of the DOOM Marine and Praetor suit in the sealed Kadingir tomb have researchers begun to put other pieces together. It is now believed the DOOM Marine might have been part of an ancient group or tribe, maybe even their leader. Whether he is god, demon or human will remain undetermined until the Argent barrier protecting his body can be deactivated. Further deepening the mystery of his origin, UAC remote monitoring drones in service during the Kadingir Expedition recorded a protracted and deadly battle during the sarcophagus extraction -- the demons attempting to defend the tomb with an instinctual ferocity previously unobserved.
Once tethered and returned to the UAC, the sarcophagus was studied and first opened at the Lazarus Labs, but went missing a few weeks later. It was believed that Samuel Hayden had the body and Praetor suit moved and hidden to keep it secure, although why he considered it to be under threat is unknown.
Of final note; to date, the DOOM Marine and Samuel Hayden are the only known non-demon entities to successfully cross over from the Hell dimension to our own, despite several attempts by other UAC human personnel to do the same.