Here's your question (reply via comment) :
If you could learn the answer to one secret (i.e. What killed the dinosaurs; Who killed JFK; Are Britney Spears' boobs real, etc.) what would it be?
To start you off, here's my answer:
What the fuck is Stonehenge? What is its purpose? Is it a temple? A sports stadium? Maybe just a pretty stone sculpture? A domino set for gigantic aliens? And while we're at it, how the hell was it built? And who did the building? (Okay, okay, so that's like twelve questions. So shoot me.)
I just saw a documentary tonight on a dude that built his own re-creation of Stonehenge, based on his interpretation of where the stones originally were. It's very pretty, all symmetrical and shit, but it didn't shed an eon of light on its purpose. I've heard speculation that it has something to do with the equinox. Why did early man care about that? Why go to so much trouble?
...now it's your turn!