GTA: Underground was more than just a map extension; it was a total conversion that integrated the assets and mechanics of several classic titles into the San Andreas engine.
: It didn't just port buildings; it included city-specific pedestrians, vehicles, weapons, and radio stations from their respective games. Custom Mechanics : download-gta-under-ground-game-free-top
While you can no longer get it from official sources, here is a deep review of what made this mod legendary and the current state of its availability. GTA: Underground was more than just a map