Jack flicked the switch on his own apparatus and looked over to the Doctor, shooting him a nod.

"Yeah," he agreed, "so I guess we follow the trail and when it ends we find out squiddy friend."

He glanced up at the light he could see above and down to the murkier waters below.

"Okay," he said with a nod, "going down?"

And on that, he turned and began swimming along towards the curve of the rock and (hopefully) the origin of the trail.