http://www.imdb.com/title/tt0418645/trivia

* The game contains several references to the original Doom games. For example, one of the magazines lying about has a "surprised" face on it - that of the Doom Marine from the first two Doom games. Also, the "player face" on the arcade machine is that of the Doom Marine.

* Within approximately 1 day of release, there was already a game modification (or 'mod') which added a light to the pistol. In the game itself, players must swap between a flashlight, or a gun.

<SPOILER>
* The name of the doctor responsible for the disaster is Dr. Malcolm Betruger. "Betrug" is the German word for fraud or deceit, which is exactly what the doctor does.
</SPOILER>

* In the original Doom, the player lost all their equipment (except for a pistol and 50 bullets) at the beginning of every new episode (for a total of two times). Although no reason was given for this effective "restart" in every chapter, the concept was carried over into Doom 3, and the player loses all their equipment (twice), on the journey to and from Hell. There are also several other holdovers from the original doom, such as a boss fight that consisted of two Barons of Hell, toxic sludge, and more.

* In one of the mails found/discovered in the PDAs lying around the base, there's a "futuristic" version of an IP address. If you "subtract" a few digits from it so it becomes a "normal" present-day IP address and perform a "reverse DNS" you'll get: "idsoftware.com", the domain/site of the creators/developers of the game.

* The PDA audio logs closely resemble the format, tone, and pacing of the audiotape Ash listens to in _Evil Dead II (1987)_

* Development team averaged about 24 people.

* Originally was not intended to be made. John Carmack, unimpressed with the current plan for the company's next game, told his co-owners that unless Doom 3 was id software's next project, he would quit.

* While most of the levels exist highly ruined, the developers first built every level as if nobody had ever walked in them. Going through each level with special developer-only 'weapons', level designers would then dismantle the levels in order to create a highly damaged effect.

* Originally, the Soul Cube was going to be split into four sections, forcing the player to defeat each section's "Guardian". Designers felt that this would make the Cube seem weak, and opted instead to keep the Cube whole.

* If you kill Larry Kaczynski and take his PDA, you will find an email from a "John Okonkwo" that turns out to be the well known "Nigeria Scam", which is apparently still assaulting inboxes well into the 22nd Century.