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...
Mother of all Xen crystals. It was guarded by at least 6-8 alien controllers, about 4 vorts, and some zombie soldiers.