Jack closed his eyes a moment and took a deep breath. Focus, because that's what he needed, and so he concentrated on how the Doctor held his arm.

And he swam.

Around him he could feel the water changing, growing thicker with the dust from the falling rocks. It was darker and harder still to see.

He didn't speak, just swam. Focussed on moving and getting out, finding a different way, down and down, lower into the depths of the cave.

He glanced again to his wrist strap, but he could barely even see it.

"Doctor," he called then, "it's too dark. I can't programme in the co-ordinates."