Hell

Hell in Doom
Hell is an important part of the Doom universe, being the source of all the monsters in the game. In the original and Ultimate Doom, all of Episode 3, Inferno, 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. Doom II's Hell levels are closer to subverted human buildings, with the exception of MAP28: The Spirit World and the last level, Icon of Sin. In Doom 3, the player ventures into Hell to obtain the Soul Cube. The reason of the invasion in all the games is experiments in teleportation and gateways going wrong.

The Hell in classic Doom appears to be a hot, volcanic planet, possibly even meant to be an alterate-universe version of Venus. Hell in Doom 3, on the other hand, is apparently in the inner regions of Mars.

Without Hell
One of the primary criticisms of the Doom novels is that Hell was cut out and the monsters became aliens genetically engineered to scare humans instead.

A similar criticism was aimed at the Doom movie, where the monsters were created by a virus and lacked the projectile attacks of their game counterparts.