"Unfortunately, nothing definitive." The Doctor chuckled. "Theories, mostly the musings of an old man. You see, whatever it was had to have given them time to build all this," he explained, his hand motioning about them. "Yet it was still devistating enough that they had to. And despite all that, they didn't keep records of what it was." He paused. "Unless, of course, those records are confidential, being kept from the rest of us. I wonder why they'd do that? Perhaps to not frighten people?"