Of course! W. Mark Felt! It was right there in front of our noses the whole time! We should have figured it was the guy that nobody ever heard of.
Meanwhile, I can't help but be a little bit sad. It's truly the end of an era: I imagine we'll never see another secret source who brings down a president and is named after a porn movie again. Although if we do, I recommend they go with "On Golden Blonde." That would be funny.