Myrtle glanced over her shoulder once, just once, to make sure the students were still following. They were in the deep, dark center of the lake now, about half an hour (as a ghost swims) from the shore. Maybe forty-five minutes in for a human.

She could see the dark blob of the underwater cave ahead and motioned for the Slytherin with the key to come forward. "Lift the key to the hole!" she ordered as bubbles spewed out of her mouth, pointing at the shell-shaped rock thing.

Merpeople were so weird sometimes. Who used a shell for a lock? A toilet-seat-shaped one would have been better, according to the ghost who lived in a loo.