Michael Phelps retired from competitive swimming after the 2016 Summer Games in Rio. He is the most decorated Olympian of all time with 28 medals, including 23 gold. But Phelps' next race, which will air on the Discovery channel, features his most challenging opponent- a great white shark!

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In an interview with Good Morning America on Thursday, he talked about the experience.

Phelps said:

“When we were in the cage, as soon as I first got in, I felt both sides of the cage moving back and forth, and I was like oh my god, we're really getting bit, the cage is shaking. They didn't look that big underwater. I guess now seeing the video, they were pretty massive sharks. I had one come literally right up to my nose, and it was crazy.”

WATCH: @MichaelPhelps talks about racing a shark on @Discovery for #SharkWeek. https://t.co/jGJ0mfxdGD pic.twitter.com/s2XBusDdT5

During testing, the American Olympian, added: ‘We were very safe. We had about 15 safety divers around.' Referring to the whereabouts of the shark, he said: ‘We weren't side by side. The challenge of trying to get a great white to swim in a straight line was difficult, because when a white attacks a seal, they come from under the surface to build speed. But I think with some of the tests we were running out there on the boat, we were able to see what they can do.'

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The 100-meter race took place off Cape Town, South Africa, and had been on 32-year-old Phelps' bucket list since “forever,” he said. “It was honestly just something I've always wanted to do, being in the water with a shark,” Phelps said. “Whether it's in a cage, or just swimming freely in the ocean. … Having the chance to literally see these animals up close and personal in their home, it topped so many things that I've done -- just being able to see how they truly are.”

The race called ‘Phelps Vs. Shark: The Battle for Ocean Supremacy' will air on Sunday, July 23, at 8 p.m. on Discovery Channel. And boy, does it look like an insane watch!