Olympic superhero Michael Phelps is one of the fastest swimmers on Earth, but he's been taking up a pretty weird challenge.
He's been racing sharks... Yes, you read that right.
Michael Phelps had a special shark suit to help him compete against the ocean predator.
And don't worry - he actually wasn't in the water with them, it was done with special effects.
Watch this to see who was the fastest...
Pictures courtesy of Discovery