"The most likely theory?" The Doctor laughed, clearly amused. He wasn't about to say what he thought was the most likely theory, but... "Oh, no. There are any number of theories behind it all, and while any one of them might make perfect sense, there's no way to say for sure which one is more likely without more information. Which we seem to be lacking." He made a tch sound. "As you said, perhaps once we reach the city we'll be able to find more."