This article has a talk page!This page contains info on data for the game Ultima Underworld: The Stygian Abyss (DOS).
TLDR: See uw-formats.txt for a description of data file structures in Ultima Underworld.
A long long time ago (before central repositories like SourceForge and gitHub), there was a hacker trying to reverse-engineer and understand the file structures and internal workings of Ultima Underworld. Notes were kept in a file called uw-specs.txt that floated around the internet for a while. Eventually, another file started floating around called uw-formats.txt that grew and grew and eventaully superceded the previous file. That latter file is still the best source of technical information on Ultima Underworld.
As of this writing (May 2025), the latest version is held and maintained by vividos at Underworld Adventures and can be viewed on github.
There are some older forks of the file that, while not as recent or complete as vividos', do have a few notes on them that haven't been merged up. See the abysmal engine version and the source forge underworld adventures version.
There is another explaination of file formats at Ultima Codex.
Finally, there is a high-level overview of some data structures in the notes page on this site.