Deep Ocean

Well, as deep as it is, it really just looks like Up. There's not much light.
Except the lights from the sub, which swivel a bit trying to get a view of the activity. Its occupant scribbles some notes, while also backing up a bit to get a better shot for the camera. And also raising up a bit. It isn't safe to make a wild rise to the surface, but it can't hurt to get started on that now.
Maybe it's some sort of alien craft? Or some lost civilization? It's certainly mechanical in some way...