"Well, variety of ways. But here, I don't know. All the light on this planet comes from the ground, they'd need incredible power to move past the Dark Star," the Doctor says. He leads her down a dark corridor to what appears, to the Doctor, to be a set of stairs.

"Down here. Least until the sirens die out."

A few steps down, however, and he finds himself waist-deep in what feels like very thick water.