A single player campaign. You must fight your way to the surface of Black Mesa...
This is some screenshots from a seemingly "cut" part - large as 2 map's - in "Ashes to Ashes, Dust to Dust". You have to "noclip" through the door near the panel opening the "A/B/C" tunnels. Some strange things here... Even fights against Black Ops. As the mod is in its "full/final version", I don't know what these map's were supposed to be, or if they were simply forgotten.
A quest that explored all those would be highly fun :3 ^_^