It seems everything not immediately explained by what is currently known about
Pre-History and /or
The Ancient World is automatically assigned some kind of Alien/Paranormal solution.
Of course there are many other more-likely possibilities. (In any event, speculation is always fun.)
Brian Dunning's excellent Overview on
Skeptoid
This is History Channel's Alien-Technology focused piece- (But it will give you a great visual tour of the site)
Later,
Paul Parducci