In a video posted to Twitter that has since gone viral, a great white shark is seen swimming on its back with its jaws open, dipping in and out of the water. The video was recorded beside a charter boat off the coast of Lincoln, South Australia. While Twitter users are amused by the fish’s “derpy” behaviour, this may actually be a shark in a state of tonic immobility.

When in this state, the shark’s muscles relax and its breathing becomes deep and rhythmic, putting it in a trance-like state. Tonic immobility occurs naturally, but can also be induced in sharks by stimulating certain pores near their snouts.

Researchers use this as a means to subdue sharks when handling them. If a shark is turned on its back, it’s thought to disorientate them enough to enter the state. However, it is still a mystery as to what purpose this state serves in the life cycle of these apex predators.

Some social media users are concerned that the shark in the above video doesn’t look as calm and immobile as tonic immobility usually presents, and may perhaps be in some kind of pain.

Seen below is footage of an experienced diver inducing tonic immobility in a shark, and explaining the fascinating phenomenon.