"I don't think it was," the Doctor said, running a hand along the top of the cave. "Look at the stone. Rough and scratched, like by claws or a machine. Not smooth like the rocks back out there. Also..."

He pressed his palm flat. "Warm. Something is pulling the heat from the water around it."

The room was bright, the lightning coming all around them. But no creature, nothing it could actually come from.

"Here," he said, gesturing ahead. He led the way deeper into the cave, keeping one hand on the wall. "It looks like it goes all the way---"

Something ice-cold wrapped itself around the Doctor's leg, and without even a second to catch his breath, he found himself pulled beneath the surface of the water.