Göbekli Tepe in Turkey is something that's fascinating to me. Estimates run between 11,000 to 15,000 years old, constructed 6,000 years before most megalith sites, such as Stonehenge, were built. Most archaeologists believe it was a temple of some kind. The odd thing that piques my interest more than anything, though, is that it is seems to have been deliberately buried! There is a lot of conjecture, of course, and there's a lot of interesting articles on it. Smithsonian did a great article on it, but it's absolutely fascinating.