One thing that I never quite understood about the back story of Half-Life, and which is unfortunately never explored in the later games, is everything that came before the events of the first game.
It appears that the folks at Black Mesa have been going in and out of Xen for quite some time. To stress this point: Humans have been 1) aware of the existence of aliens, 2) developed teleportation technology, and 3) visited and looted the world of Xen, and all this for quite possibly a considerable amount of time.
Now, there’s no indication that “normal” people in the world of the game are aware of any of these facts. To put it in realistic terms, any single of those items would come to a real person of the early 2000s (as well as to us in 2012) as a profoundly shocking piece of news, comparable to the actual discovery of Atlantis, or perhaps even the actual return of the Christian messiah. It would be hard to imagine something more astonishing.
Yet somehow, a pretty sizable number of people must be in on all this. How does that work? How come nothing of this leaks out?
Now a more important point: The HECU military seems to have been going in and out of Xen as well, as we can tell from all the leftover military equipment on Xen (e.g. the tank seen during the end sequence). But that can only mean that the HECU and the Black Mesa scientists have been collaborating for quite some time, too! Does that mean that a permanent HECU detachment has been installed at Black Mesa all this time? That would explain why there are several garage-type buildings in place that seem to be designed to service tanks.
That would also explain why the military was so quick to arrive on the scene. On the other hand, it would be quite at odds with the initial airlift sequence of Opposing Force (although Adrien Sheperd could have been part of the reinforcements coming from further afield), but also with the callous willingness on part of the soldiers to slaughter civilians. If they’d been living and working with the scientists for a long time, would they not have some reservations against shooting scientists at point-blank and dumping the bodies into the sewers? Don’t marines have some sort of code of honour?
Anyway, in short, my question is how much everyone had been in on the whole alien invasion thing before the resonance cascade even happened. Also, I suppose, when Gordon re-emerged in HL2, why didn’t he stop and ask any one of all the ex-BM people what had happened, and how long people had know how to teleport around, and what else they were keeping from him?