Alright, time to finish this. As I was saying, the Yautja more than likely held the coming of age ritual inside if a pyramid on purpose so as to seal in the Xenomorphs and prevent them from escaping into and infesting the main body of the city should things get out of hand. All of a sudden though, the ritual doesn't go quite according to plan. The Aliens bust out of the pyramid and immobilize the entire city's population to be used for hosts. Lets say five Predators had been hunting the Aliens, two have been killed, leaving three left who have escaped to the top of the Pyramid. With nearly thousands of people exposed to the facehuggers, this in turn results in thousands of obviously pissed off Aliens swarming up all sides of the Pyramid to get to the remaining Preds. Realizing that they have no chance, the two Preds still alive ( the third is dead after getting impaled by an Alien tail ) activate their self destruct and KA - BOOM! The Aztecs are effectively wiped out along with all the Aliens! With that training ground gone, the Preds must find another spot on Earth so that the ritual can continue. I must credit Adam JZ for this brillitant theory, but the Preds then pick Antarctica, the harshest environment on Earth, isolated from all the rest of the world. This isolation is crucial, and the Preds know this is important because they dont want the Aliens escaping to infest the rest of the world and spread out of their control again; like what happened in S. America. The Preds then construct the ritual Pyramid out of their own technology with the help of their surviving worshippers. At the 1,000 year mark several humans are sacrificed leaving behind the eerie skeletons and such that you see when the team enters the Pyramid. Now, 1,000 years later, its time for the ritual to begin again. Now someone mentioned of just how convenient the Pyramid was in a hollowed out cavern when the team finally is able to drill that deep. In one of the featurettes you hear Weyland say "7 days ago one of my satellites discovered a sudden heat plume over Antarctica". My guess is, you have to remember now, this Pyramid is mostly Predator technology, so, perhaps the Preds planned for the fact that in a 1,000 years of idleness their pyramid would be buried underneath layers of ice, so they built in some form of device that heats up the outside of the pyramid, melting the immediate ice around it, creating that cavern making it at least somewhat accessible. Is anything I'm saying making any sense, I hope so, my fingers will be pissed if I they knew they typed so long only to have their hard work backfire.