Mary Ann has, at this point, figured out a way to converse silently. She pulls out her phone and types into the notepad: It must be bigger than a block of flats.

Or at the very least that size. And she doesn't like that thought. Because... she can't quite pinpoint what is directly above them right now, they walked for long enough underground that can't even make an informed guess any more. But Ben tracked their path, didn't he? Maybe... Can you tell what's above us?