Professional surfer Mick Fanning was taking part in a special event in Jeffrey's Bay, South Africa.
However, just as he'd completed his set, he noticed something swimming underneath him.
The incredible footage, captured by television crews, sees a shark swimming underneath him and flashing up for a few seconds.
Here's the full seven-minute video of what happened afterwards, including the mass evacuation of the water.
Take a look.
Thankfully, Fanning was unhurt and escaped injury. The event itself has been shut down for the day to allow an investigation and to see if the rogue shark will turn up again.
Pretty crazy stuff, we think you'll agree.
Via YouTube