What is the most mysterious archaeological discovery ever made?

Archaeology has always been shrouded in mystery. There's just something about the process of digging up the past that is a bit spooky and mysterious. Some digs have been more so than others though. The most mysterious one has to be the Clava Cairnes. While some may argue that this finding is not the number one mysterious discovery, I disagree.

The stones were found in a perfect circle for no apparent reason. There has to be more to it. They didn't form by themselves. Many think it is an ancient burial site while some think it is a celestial figure of some sort. A number of people think it is from the aliens. No one may ever know the truth and that is what makes it so mysterious.