: Helluva sentence there. I'll start with guessing: The Marathon Scrapbook -
: Nar history
: Stuff removed from the original game, such as Hounds, the MOW-MDN, also
: arrival.bungie.org.
: Cryptic Usenet posts like Greg's "What are all dreams about
: (non-Freudian)?"
: The Lost Network Packets
: The Pfhor Mating Ritual
: The snail mail letter confirming that the player is a cyborg (no new
: information there)
: Duality previews (to some degree, maybe you can get another perspective on
: the whole eternal-warrior mythos/philosophy)
: Other cryptic messages like Matt's one about the Great Devoid, and Nathan
: Bitner's one about Tau Ceti. But keep in mind, most of that complexity is
: unintentional, because it was so slap-dash (I'm paraphrasing Matt).
: I wonder if there is, or should be, a comprehensive list of such sources.
This is still nowhere near the amount of stuff in the GURPS manual. I'd say more than 50% of it is not canon. (Not that that's a bad thing, variety and breadth of content is one of the most important aspects of an RPG, and the Marathon universe was never meant to be one in the first place.)
