Transcript for Michael Phelps on preparing for his race against great white shark
Super swimmer Michael Phelps is the most decorated olympian of all time. But his next opponent literally smells blood in the water. A natural since birth who's never needed coaching and eats people like Phelps alive. So is he scared? He should be. Here's ABC's T.J. Holmes. This isn't even close! Michael Phelps is swimming away from everybody! Reporter: No man has been more successful in the water than Michael Phelps. New world record! Reporter: The swimming legend retired after last year's Rio games. As the most decorated olympian of all time with 23 gold medals. Michael Phelps has done it again! I'd raced the fastest swimmers on the planet. Except for one. Reporter: But this weekend in a once-in-a-lifetime event the 32-year-old golden boy is coming back for one final race. Am I faster than a shark? Reporter: Wait. Did he just say shark? No, we're not crazy. It's billed as Phelps versus shark. You can't actually outswim a great white shark, can you? We'll have to see. Come on. We'll have to wait and see. Reporter: Come on now. The spectacle billed as man versus mega fish was put together by the discovery channel for shark week, the yearly week-long homage to our cuddly sea friends, the sharks. What would you say your relationship with sharks has been up until you went through this process? Fascinated. Racing a shark or being in the water with sharks was something I always wanted to pop. Reporter: I know what you're thinking. How is this even possible? Surely they're not going to put the golden boy -- That's insane. Reporter: -- In open water right next tie greo a great white. A shark expert explains. The show is Michael going on a journey to South Africa to see great white sharks and for us together to try to see how fast great white sharks can actually go. And we would compare that to Michael's speed. Discovery is saying we want you to race a great white. What did you think when you first heard that? The first thing that popped in my head was safety. I mean,hat's the number one concern now, being a father and having a family. Once we figured out that it was safe for us to do it, you know, we weren't side by side. It's not like I was racing a shark neck and neck. Reporter: Let me be clear. Despite the punchy music and pumped-up title Michael Phelps is not racing side by side with a great white shark. Nobody's that crazy. Did that cross your mind for a second, is this what these people want me to do? Soon as I heard the idea, I was like there's no way they think I'm doing it side by side, right? First time I'm diving with great whites. We didn't do it in a swimming pool. Reporter: How did you all ensure safety for this thing? We had 10 to 14 divers underneath us. We obviously can't judge where a shark's going to swim but thankfully we didn't have any visitors come up. And yeah, it went perfectly. This is going to be awesome. There's one on your right, guys. Coming in here. Michael, guys. Coming in on the bay. ??? That was a shark. I saw that one. Saw that one. Reporter: Phelps says he's trying to raise awareness to get the message out that sharks are not the man-hunting beasts we know from "Jaws." You know, I think a lot of people really only think of sharks as predators and aggressive and all they want to do is eat us. And you know, hopefully with the shows that I was a part of here, you know, we can get the message out that it's not the case. Reporter: But does the greatest of all time stand a chance against the titan of the deep? Let's match them up. Phelps is 6'4", weighs in somewhere around 200 pounds. The great white shark, meanwhile, can be between 11 to 21 feet long and can weigh over 2,000 pounds. Not really a good start for team human. Great whites are just a marvel of evolution. They are built for speed. Their body shape is built like a bullet. That makes them super hydrodynamic in the water. And they've got this huge tail that's built with a shape like two symmetrical crescent moons. Bullet-shaped body, huge tail. Okay. A 5,000 pound shark can swim 25 miles an hour. Top speed 25 miles an hour. What's the fastest you've swum? The fastest I've done it without fins is probably like 6. Reporter: Phelps is enlisting a little technological help. This is a step above the monofin. This is to mimic the white shark's tail. It's got something called a hollow keel, a structural support that buttresses the tail that allows extra power and strength. Just putting on this fin is going to make me the fastest. Yeah. I should have done this all along. That was a little transition that I had to work into, but I think we came up with a way to swim the fastest and you guys will see it on Sunday. Reporter: Two people who will definitely be watching, Michael's wife Nicole and their 1-year-old son boomer. I mean, she was stoked too. She wanted to get in the water with them too. Reporter: Do you want him to be a competitive swimmer, boomer? If he wants. You know, I'm definitely never going to pressure him to do it. Reporter: But it would be cool to see, wouldn't it? It would be awesome. He does like the water. He enjoys being in there. He's very comfortable in it. He knows how to blow bubbles, ck. But if he's truly passionate about it, then great. I'll help him along the way but definitely not something I would pressure him on. Reporter: Even though he says he's retired from competitive swimming Phelps insists that practicing for this shark race is not practice for the Tokyo olympics. You've repeatedly said that 2020 is out. Do you think you would be physically healthy enough to compete in the three years if you wanted to? I think if I wanted to I could. I don't think -- I'm not going to say can't. Even though I'm out of the water and retired from the competitive side of it, I love swimming. It's going to be a part of my life forever. Reporter: Do you reserve the right to change your mind about 2020? No. I mean, for me like I just really have no desire. Reporter: So if this is his last race, for real, will we see the great one win gold against mother nature? We'll all have to wait to find out. For "Nightline" I'm T.J. Holmes in New York. "Phelps versus shark" airs this Sunday on discovery's shark week.
This transcript has been automatically generated and may not be 100% accurate.