24 July 2017 Last updated at 14:56 BST
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
Olympic superhero Michael Phelps is one of the fastest swimmers on Earth, but he's been taking up a pretty weird challenge.