3DRealms DMU Pack... this is conversion of BellToll (the fixed version) and Ascension (with added bot paths because it didn't really support bots). The secret tunnel doesn't advance you by default anymore on UT (we should have fixed that but if you move forward you're okay), but you can get unstuck easily.

These levels can be seen as the "bonuspack" by 3DRealms, by Keith Schuler. When they licensed the engine for Duke Nukem Forever. In fact DNF2001 has a sequel to the BellTolls map but strangely in a technological theme as opposed to the "wild west" theme seen in the game at some point during the Nevada maps.

The BellToll map shares a title with the ChaosUT GOTY map, but its a very different map in practice. Both maps feature a gimmick with unlocked secret(s).

This was kind of a 3DRealms addon thing, every Unreal engine licensee basically had the right to produce addons be it free ones or paid expansion and this was their way of doing that. Many of the licensees had a cancelled planned addons, but with their titles cancelled/delayed also went the Unreal addons down the drains...

- Leo