No, it's not a still from Sanctum - although James Cameron's new diving film was inspired by a near-catastrophic expedition by the man who took these pictures, Wes C Skiles.
This is a real photograph from one of the most mysterious environments in the world. You could visit Abaco Island, in the Bahamas, and have no idea that beneath you lies this vast network of caves, accessible by as many as a thousand 'blue holes' - submerged vertical caves peppered with entrances to this forbidding
Of the 1,000 or so blue holes in the Bahamas, fewer than 20 per cent have been investigated, and almost none fully explored
Exploring these passages is the diving equivalent of climbing K2 - you've got to be exceptionally well trained and well prepared. Even then, safety is no guarantee; Wes C Skiles died during a dive last summer.
Sanctum is dedicated to him.
Divers maintain a taut safety line at all times. Without it, it could be nigh on impossible to find your way out before your air supply runs out. Divers carry three tanks of nitrox mix - one to use on the way in, one on the way out, and one for emergencies - and three lights, which are used to communicate as well as navigate.
Standard practice states that if any one light fails for any diver, the whole dive is called off.