Years and years of devoted play and source code study have yielded many of Doom's secrets, but perhaps the most unusual is a phenomenon named the ghost monster. When a monster dies by crushing, or has its corpse crushed, its remains are converted into a puddle of gore. These remains can then be resurrected by an Arch-Vile, which was only meant to raise intact corpses.

Ghost monsters earned their name from their ability to drift through walls, and in rare cases, floors and ceilings, despite Doom's engine not actually supporting three-dimensional "room over room" architecture. Persistent ghosts are capable of chasing a player through any barrier or door in their way. In addition, they are indestructible by conventional attacks and weaponry. Combating ghosts requires setting elaborate traps, often difficult or impossible without levels designed for it. Even if killed, a ghost can always be resurrected again and will retain its properties. Some extremely rare situations, not yet isolated, result in every monster in the level simultaneously gaining ghost status and becoming alerted to the player's current location.

Players and level designers have been exploring this effect for years, triggering it in both commercial releases and fan-made expansions. Particularly devoted players claim to have found techniques for harming ghosts that rely on counting animation frames or attempting to trigger other bugs, but despite individuals racking up thousands of hours of practice, no single method has been proven, and programmers have reported directly conflicting results on examining the code for hints.

Custom levels featuring ghost monsters are more popular now than ever before. Some players have reported that older ghost levels no longer run regardless of the game version used, or alternately, that their exploits no longer bypass the ghosts' invulnerability. Most of the recent ghost levels are panned by reviews as exceptionally repetitive, with unsettling level design and droning music, but ghost aficionados are willing to put up with them despite the high rate of computers freezing or suffering total crashes.

"Ghost hunting" discussions inevitably result in furious flame wars, and in recent months the tight-knit community's sites have suffered increasingly frequent shutdowns to repair virus attacks and delete controversial levels. Site administrators have been contacted by several local authorities regarding harassment, death threats, and missing persons cases.