So after killing more soldiers than I can count, and frankly getting sick of them, I find one of those nuclear warheads, like in Opposing Force. Upon approaching it to disarm, it goes off, but instead of dying, I get teleported to Xen. Only now, I come across some ancient ruins...
In the Half-Life universe, death involves getting a train ride through Black Mesa.
This time I'm apparently leaving it though. At the end I get approached by a Barney with G-man voice, basically telling me: don't try to interfere, bitch.