Hellgrowth is the name given to the flesh-like substance that appears in areas corrupted by Hell.
The earliest version of hellgrowth, before it was really solidified as a concept, is usually represented by various textures such as SKINFACE, SKSPINE, and SKSNAKE, as well as the corresponding flats. The first instance is found in E2M3: Refinery in the moving pillar room, where organic columns constantly raise and lower. E2M4: Deimos Lab, E2M5: Command Center, E2M6: Halls of the Damned, E3M1: Hell Keep, E3M3: Pandemonium, E3M4: House of Pain all feature additional examples. In Doom II, it can be found in MAP08: Tricks and Traps, MAP16: Suburbs, MAP20: Gotcha!, and especially in MAP28: The Spirit World.
Hellgrowth is introduced in Administration, shortly after Sergeant Kelly tells the player to watch out for some kind of "unidentified growth" that is "taking over the base." It is prominently featured in Transfer Station, Delta Labs 3, Delta Complex, and Processing Distribution Center. It is also encountered in the Resurrection of Evil expansion, particularly Phobos Labs and Delta Labs.
Hellgrowth is given its canonical name as well as some lore explaining its purpose in the Hellish corruption process besides being disgusting scenery. Special forms of it, such as gore nests and tentacles, are also introduced.
Doom Eternal explains that hellgrowth is a cancerous, rapidly growing organic biomass that is associated with demonic activity and invasions. Gore nests are explained to emit powerful electromagnetic fields which induce an inverse quantum field. This results in tears in space-time which open up portals—also called continuum gates by the ARC—to Hell. Furthermore, it is mentioned that hellgrowth in general emits toxins hostile to native life, effectively terraforming the environment around it into something suitable only for habitation by demons.
In the Reclaimed Earth level of The Ancient Gods, Part Two, which occurs after the invasion of Earth has been thwarted, hellgrowth can be seen to have fossilized into a stone-like substance in the absence of an active invasion. This suggests that a constant connection to Hell is required to sustain it.