The Doctor swam over, picking a piece of the rock up and crumbling it with his fingers. "This looks like granite," he said. "Or, well, something like it. But all the chemical components have been stripped, drained."

He turned his head, glancing back towards the cave entrance. The likelihood that this creature was only around to help was getting slimmer and slimmer. If they had to, they could destroy it. They would destroy it. Jack's world would be safe.

He looked up, then nodded out to the water. A bait ball of the small, alien fish swam through the rocks, spinning faster and faster in fear.

"Strange," the Doctor said. "That sort of thing only really happens when a predator is attacking."