Michael Phelps is diving right back in to competitive swimming.

He's the world's most decorated Olympian with 18 gold medals.

And though he announced his retirement following the London Olympics, he'll be competing again starting Thursday.

(SOUNDBITE) (English) FORMER OLYMPIC SWIMMER, MICHAEL PHELPS, SAYING:

"I'm signing - not that I know of - any contracts. I'm doing this because I want to."

For starters, he'll be swimming in Arizona's 2014 Arena Grand Prix.

But there's speculation this will lead to an Olympic return for the 2016 games.

(SOUNDBITE) (English) FORMER OLYMPIC SWIMMER, MICHAEL PHELPS, SAYING:

"I guess I'm at least going with my mom, whether I'm in the pool or in the stands I guess time will tell."

The 28-year-old Olympic legend admits he had fun during a two year hiatus. So much fun that he gained 30 pounds.

(SOUNDBITE) (English) PHELPS' COACH BOB BOWMAN SAYING:

"Well when he first came back, he was so out