What's the worst codebase you've ever worked on?

You know the kind I'm talking about.

A huge lumping monolith of spaghetti with barely any tests, class hierarchies a mile deep, and a list of dependencies that can make a grown man weep.

Written by a self proclaimed "genius" 15 years ago with junior devs patching features on top of it since.

What's your story? How did you deal with that mess? Are you still on that team?