Thanks to .RK we know about how large the Facility is (and the answer is, surprisingly, not that large) but his maps are 2-dimensional and for the longest time I’ve wondered just how deep underground the BMRF actually extends.
What seems like a logical point for the deepest section of Black Mesa would be either the bottom of the big shaft the slime river drains into in the last map of Blast Pit, or the lowest section of On A Rail’s rail system. It’s difficult to determine exactly which, since the pipe Freeman uses to enter Power Up is a long way up the big shaft, but to get from Power Up to On A Rail there is an elevator going down a significant ways.
Fortunately, it’s pretty easy to open up the maps in hammer and take out elements of the geometry corresponding to level transitions, then lay each one so that it overlaps the corresponding element from the next map while preserving the height relationship between them; thus building a ‘chain’ of height-accurate structures all the way from the surface warehouse down to the bottom of the shaft in Blast Pit (and then extend that to the bottom of the elevator shaft in OAR:
It turns out that the slime shaft is deeper than the OAR sections, but only by about 250 Hammer units. Furthermore, the distance from the top of the elevator shaft in WGH (which is at ground level) to the bottom of the slime pit is 8580 Hammer units, which is equivalent to 6435 inches, 536.3 feet, or 163.46 meters.
That’s not actually very much. The Cheyenne Mountain underground missile command center goes more than 600 meters deep, the TauTona mine in South Africa goes down nearly 4 km, and there’s even an open-pit mine in Utah that’s just a big roofless hole dug to 900 meters. In fact, the cliffside area in Surface Tension extends nearly 35,000 Hammer Units down to the desert floor (6144 from main level geometry, plus 1796 * 16 from the skybox), meaning that Freeman’s Mind was absolutely correct when it claimed that Black Mesa was in fact physically built into a mesa. This works out to about 2,100 feet, roughly twice as tall as the mesas in Monument Valley EDIT: SEE BELOW.
I considered whether the canals in Unforeseen Consequences might actually run deeper than the slime shaft, but it’s hard to say on account of the elevator ride of unknown length that occurs between them and Office Complex.
In Forget About Freeman the player goes down two relatively short elevator shafts to get to the Lambda Complex, and then spends most of the rest of the level climbing upwards, so I legitimately suspect that many parts of the Lambda facility are in fact located ABOVE the ground-level that is seen in c3a1b.