This is a working Pre-Disaster version of the Black Mesa Singleplayer chapter "Unforeseen Consequences", after you leave Sector C and enter Sector B.

[TO LOAD THE FIRST MAP: OPEN DEVELOPER CONSOLE (~) AND TYPE "map pd_c1a1c" WITHOUT QUOTATIONS]

Imagine the day before the Resonance Cascade - scientists are working and Gordon (for some reason) now has unlimited access to all parts of the facility! Here we have the maps of Sector B that have been un-disaster-ified from the singleplayer campaign to the day before the Resonance Cascade!
There are additional areas too, and still more to be added.