Hell is a cornerstone element of the Doom universe. It is the source of the demonic invasion in the games. It has a significant presence in many levels, and is the setting of parts of Doom, Doom II, Final Doom, Doom 64 and Doom 3.
Typical features of Hell in these games include copious numbers of mutilated bodies, some apparently still alive and presumably of the damned, and scatterings of Satanic iconography. Most of Hell's levels' architecture involves jagged rock walls, fire, wooden doors, stalagmites, dead trees and lava in place of nukage. Rarely does the player come across natural or technological aspects in the Hell levels, and indeed the structure of the levels themselves do not have any sense of flow or continuity.
In every Doom game Hell is depicted as having developed (or stolen from Earth and made quick use of) biocybernetics, implanting modern and futuristic weapons into many of its demons, in fact the cyberdemon and the spiderdemon are both powerful demons with mechanical and organic body parts, and the icon of sin appears to be an enormous, partially mechanical demonic head, though Doom II's endgame text indicates it has a suitably massive body as well.
 Inhabitants of Hell
Hell is populated by a wide variety of creatures, from damned souls and zombies to demonic beasts and cybernetic monsters. Not much is known about the exact origins of these creatures. Some of them are likely former humans, while others may have been born (or created) in Hell itself. These creatures are all savage, brutal, and relentless, yet fairly intelligent and able to work together towards a common goal: The conquest of Earth.
In Doom 3 the creatures of Hell are controlled by an entity of great power and malevolence, which refers to Hell's inhabitants as its "children." This nameless entity speaks through Dr. Malcolm Betruger and later manifests itself as the Maledict. It is unknown if this entity is the sole leader of Hell, or just one of the leaders of Hell. In any case, the creatures of Hell obey its every whim. Doom 3 also establishes that the demons once controlled Earth at some point in the distant past, but for unknown reasons lost possession of it. Since then, they've sought to take it back and destroy the Human race.
 Demonic possession
Zombies in the original Doom are often regarded as "former" humans possessed by demons from Hell. The Doom Bible states that possession can happen to living but sleeping humans.
In Doom 3 and Resurrection of Evil, it's demonstrated that both dead and living humans can be possessed. Most of the civilian and security personnel on the Mars base are possessed and converted into zombies in the first wave of the demonic invasion. These zombies also lack a soul after a given amount of time, as the Artifact cannot absorb a soul from a downed enemy that was a zombie, although it can take souls from any other bodies and in one instance it can take the soul of a man during the game who becomes a zombie in front of the marine and is killed.
For unspecified reasons, some characters - such as the Marine which the player controls - are not taken over. Counselor Swann, Jack Campbell, Sergeant Kelly, along with a handful of marines and civilians are also amongst those not instantly converted into zombies. Throughout the game, the Marine experiences instances of blurred red vision - usually accompanied by evil laughter, or whispering, or demonic language. Also, early in the game, when the Marine looks into a mirror, he sees a vision of himself starting to turn into a zombie. These incidents are possibly the result of the demons' unsuccessful attempts to possess him. These events, coupled with the fact that quite a few of the base's military personnel were initially unaffected by the demonic invasion, suggests that the demons cannot possess those with strong, well-disciplined minds - or at least have difficulty doing it.
A second process of zombification is also referred to in Doom 3, in which the victim is slowly driven insane by a mysterious ailment. The victim suffers from hallucinations and voices, and becomes increasingly hostile and violent towards those around him. Physical changes occur only at the end stage. It is unknown whether this is caused by a virus or magic, or is a side effect that Hell has on the mind. Initially, this insanity was only contracted by those who visited Hell during the Delta Lab experiments. However, in the weeks prior to the invasion, other people in the base - who had no connection with Delta Labs - began experiencing symptoms as well. It is likely that Dr. Betruger was behind the spread of this psychosis, as part of preparing for the demonic invasion.
For reasons not stated, the Player is not afflicted with this insanity despite journeying through Hell to recover the Soul Cube. This may be due to him having a strong mind. Alternatively, the Soul Cube may have protected him from Hell's influence.
In the classic Doom novels, where Hell is not a part of the story, it is indicated that only dead humans can be made into zombies, through biotechnology. In the Doom 3 novels, where Hell is part of the story, many of the UAC personnel are possessed just like the game, but some zombies grow claws, jaws and demon-like body parts and others are stronger and faster than the game's zombies.
 Access to Hell
In the classic games Hell is discovered following experiments in teleportation technology, and during gameplay Hell is only accessible by long-distance teleporters or gateways. In the original Doom teleporters notably have Satanic symbols on them.
Hell in Doom is heavily based on the religious concept that the souls of bad people spend eternal damnation in Hell after death. This is indicated in the Doom II endgame text, "You wonder where bad folks will go when they die now.", and again in The Plutonia Experiment's endgame text, "Hell has gone back to pounding bad dead folks instead of good live one". Because one cannot physically travel to Hell without the aid of some sort of gateway, Hell's actual location is never revealed, though it is indicated by the Doom FAQ and by the unusual behavior of the environment in Hell in Doom 3 that it is not a part of our universe but rather another dimension entirely. On the classic games, the only way to get physically out from Hell is killing the leader of the demonic invasion such as the Spiderdemon or the Icon of Sin.
In Doom 3, it transpires that teleporter technology was derived from tablets left behind by an ancient Martian race, which went virtually extinct after a full-scale demon invasion on Mars some point in the distant past. Unlike in the original Doom where Hell was discovered upon the invasion, in Doom 3 the UAC on Mars was secretly aware of Hell's existence before the events of the game (though they didn't actually know it was Hell until much later) and actually sent expeditions there to capture demons for study. Dr. Betruger became obsessed with the other dimension and eventually visited it himself, and became corrupted as a result. He later arranged for the demons to invade the Mars base through the main teleporter in Delta.
Doom 3 also establishes that the demons can create their own portal - known as the Hell Hole - which the Player must destroy in the final level. In Resurrection of Evil it is revealed that the demonic Artifact found at the beginning of the game acts as a gateway between Hell and our reality, and upon being awakened, enables the demons to invade Mars again. At the end of the game, the Artifact is destroyed, along with the Maledict, thereby cutting Hell off from our dimension once more.
 Differences between games
While the major Doom games all depict Hell, the theme changes slightly between games. The variations between Doom and Doom II are based mainly on levels created by different designers. Doom 3's Hellish atmosphere departs more strongly from the original games.
The original Doom's Episode 3, Inferno, entirely takes place in Hell, where it is depicted as predominately brimstone-covered with a fiery sky, complete with a demonic cathedral and pools of blood. Walls and floors occasionally appear to be made from body parts including human skulls, intestines, spines and skin. In the PlayStation version, Hell's sky is filled with flames, while the Saturn version's stages have a city skyline.
The game's box art and the ending screen for The Shores of Hell both depict rocky, barren landscapes.
Thy Flesh Consumed takes place on Earth immediately after the player's return from hell, as evidenced by its endgame text and the episode's resemblance to Inferno's ending sequence. Until the episode was released, the first level to take place on Earth was MAP01: Entryway (Doom II).
In the original Doom, Mars's moon Deimos provides the first link between here and Hell.
In the storyline approaching the beginning of Doom, military experiments are conducted between the gateways at UAC facilities on Phobos and Deimos. Something went wrong, and "soon afterwards, Deimos simply vanished from the sky."
Deimos's mysterious absence is referred to in Knee-Deep in the Dead's ending text, after the player steps through the gateway at the end of E1M8: Phobos Anomaly: "It...looks like the lost Deimos base." It is later revealed at the end of Episode 2, The Shores of Hell, that the entire moon had somehow been transported to Hell, which would no doubt account for the complexes being seemingly warped and taken over by demonic means more so than the Phobos installations. The gateways, still functioning between Phobos and Deimos, provided the first entryway into Hell.
 Doom II
Doom II's Hell levels are often closer to subverted human buildings, with the exception of the last three levels. However, map 28 is not exactly Hell, but rather the pathway from Heaven, Earth and Hell, like Limbo (hence the name). The sky in these levels (where there is any) contributes heavily to the Hellish atmosphere.
 Final Doom
Final Doom's portrayal of Hell does not deviate much from previous depictions, and seems to be a combination of the original Doom's cavernous areas and Doom II's building-strewn stages. Hell levels have two different skylines; a "nightmare" red sky in TNT Evilution and a crimson sky that looks like stretched, bloody muscle (the end of level tally screen background) in The Plutonia Experiment.
 Doom 64
Doom 64's Hell levels take a dark, cavernous, and frightening approach. There are two separate types of Hell environments: rocky, volcanic areas with a burning red sky and mountains, or similarly-themed mountainous areas with dark blue storm clouds, complete with thunder and lightning. The architecture found in most of the levels resembles castles, cathedrals or temples, replete with vicious Satanic and horrific symbolism, including plentiful pentagrams, inverted crosses, and blasphemous altars. Various human remains are strewn about these levels, including impaled heads, butchered carcasses on meat hooks, and splattered corpses on the ground. Later levels possess a skyline of burning red or green fire against a dead, black sky.
 Doom 3
In Doom 3, the player ventures into Hell to obtain the Soul Cube. Hell is largely a claustrophobic and cavernous plane with crimson blood sky (it also resembles outer space to some extent), both dark and fiery (although "outdoor" sections do appear). Mostly, Hell takes place in a large castle-like area, furnished with massive stone bricks, broken cell gates, cave-like passageways, towering obelisks and columns, glowing Satanic glyphs and pentagrams, and huge oceans of magma. The conventional rules of physics are frequently violated, with the giant stone bricks moving on their own accord to form bridges or access ways. Supplies left behind by the UAC's secret expeditions are scattered throughout the level.
Unlike in Classic Doom, the atmosphere in this Hell is almost constantly noisy, typically with the crying and moaning of damned souls and extreme sizzling and bubbling of hot magma. People who enter Hell tend to experience a surge of physical energy much like that of an Adrenaline rush: seemingly able to continually exert themselves without tiring. Dubbed the "Hercules Complex" by researchers, this effect is noted in the game by the player having an inexhaustible stamina bar (with flames shooting through it) while in Hell.
Also in Doom 3, Hell corrupts parts of the Mars base in the form of a fleshy, tentacle-like growth, often oozing with blood and in some cases covered in a chitinous exoskeleton with bone-like spines protruding out. This demonic growth is first seen in the Administration Complex, and the Player is alerted to it by Sergeant Kelly. The growth's presence is initially subtle, but becomes more and more apparent as the game progresses. By the last third of the game, the demonic growth has literally consumed entire sections of the base, and even transformed some corridors into pathways of hovering floor tiles suspended over pools of lava.
 Doom 3: Resurrection of Evil
In Resurrection of Evil, the combat engineer Marine takes a journey to Hell after activating the Phobos Labs teleporter, which leads him to the old Delta Complex from Doom 3. As the Delta Labs and other Mars City sectors seem to be dominated by a demonic vortex from the main gateway, the Marine gains access to Hell to return the Artifact in order to stop the invasion.
The area of Hell this marine visits differs drastically from that in Doom 3, and includes many disjointed and varied areas. The majority of the level consists of a long series of stone caverns, occasionally interrupted by stone hallways. Other areas of note include a green foggy area where the marine is forced to use the Artifact to destroy a large number of demons, and the area of the final battle with the Maledict, an eroded island with sizzling magma and a gigantic demon skull resembling one found near the entrance of Hell in Doom 3.
Somehow, like in Doom II and Final Doom, when the combat engineer Marine enters Hell, he manages to keep his inventory with all of his weapons upon entering. However, the marine's fate remains unknown at the end of the game due to its ambiguous ending.
 Lost Mission
The Lost Mission includes two new Hell levels which draw inspiration from the previous Doom 3 levels as well as the original Doom. Notable landmarks include a forest of crooked mockeries of the Christian cross, a gigantic obsidian fortress resembling the Unholy Cathedral which floats atop a conflagration of lava and fire beneath a swirling chaotic vortex, the river Styx itself, and an outpost somehow built in Hell by the ancient Martian civilization, inside which Betruger had built the other half of the experimental Exis teleporter. This teleporter appears to be directly integrated into the remains of ancient civilization technology, suggesting they may have originally used the site for the same purpose of bi-directional teleportation to and from Hell.
 Doom 3 Novels
The atmosphere of this Hell is very similar to the game, with claustrophobic sections and cavernous planes. But there are more damned souls and possessed human bodies than in the game. The place where the Soul Cube is guarded is changed to an enormous throne which possibly belonged to the Cyberdemon. The Guardian of Hell is also absent, replaced by an enormous horde of demons. John Kane's escape from Hell is also longer and more difficult than it is in the game.
 Doom without Hell
One of the primary criticisms of the Doom novels is that Hell was cut out and the demons became aliens genetically engineered to scare humans instead.
A similar criticism was aimed at the Doom movie, where the monsters were humans mutated by a Martian gene splicing experiment, and universally lacked the projectile attacks of their game counterparts.