"Morgana, quiet," the Doctor says, holding up a hand. "Can you hear that?"

It was the sound of running water, something big running water. He looked back to make sure she was following, and then began to head out, to investigate the sound. Did they find the sewers? Were they under the city? How did they end up here? How did the computer get that sort of access?

"Be careful," he called back. "There could be more traps."