You guys all need to watch "Ancient Aliens"......it's a whole series that's on....I think the History Channel. It discusses this very topic. Some of the episodes are a bit hoaky, but many of the episodes cover some convincing topics that really make you think.
Some of the stone work they show on there is all but unexplainable. Huge stone walls constructed from single stones that are about 3/4's the size of my garage, and they are carved perfectly to fit against all the surrounding stones.....and they are NOT square cut. Each rock is a fairly random shape without any flat or straight sides, and it is carved so precisely to fit all of the stones it's stacked on, and that are stacked on top of it, that you can not slide a piece of typing paper between them. Considering that most of those stones weigh in the range of 50 to 100 TONS, it's tough to explain how humans could have carved the stone so precisely, moved them once they were carved or put them into position on the wall so they interlock with all of the other stones. It would be virtually impossible for us to do anything like that today with modern equipment.
