Michael Phelps has huge hands and feet, a powerful torso, and the competitive drive that earned him 23 Olympic gold medals in the pool.
The great white shark evolved into an aquatic speed demon over millions of years.
What happens when the two race head-to-head?
The Discovery Channel is airing the results of this unusual experiment at 8 p.m. Sunday in an episode of Shark Week. But two Philadelphia-area researchers say the outcome is in no doubt.
"He definitely does not have much of a chance," said Brian Sennett, chief of sports medicine at Penn Medicine.
"Michael Phelps is going to get ripped," said Frank Fish, a fittingly named West Chester University biology professor who studies aquatic propulsion.
Ordinarily, Phelps, 32, can achieve 5 or 6 mph in the pool. For the race, he wore a sleek wetsuit and a monofin, which added a few miles per hour to his speed.
Fish and Sennett said three main factors make the shark a speed machine:
Shape. The big predator has a streamlined hydrodynamic profile that is ideal for streaking through the water, almost like an elongated teardrop, Fish said. The bodies of birds are similar, reducing the amount of drag as they glide through the air. Humans, on the other hand, have a "bulbous" head and wide shoulders that interfere with smooth flow, said Sennett, who went into greater detail in a University of Pennsylvania blog post.
Muscle and a tail. The back half of a shark is rippling with muscle, allowing it to displace huge amounts of water with its stiff tail, Sennett said. Ditto for whales and dolphins. "If you think of how much water you can splash someone with, and you think of what a whale or dolphin can do at SeaWorld and splash the whole first several rows in the amphitheater, you quickly realize how much water they can push with their tails compared to what we can push with our hands," Sennett said.
And the shark's speed is not just about pushing against the water. Fish said the tail is shaped in such a way that it also generates lift, much like an airplane wing.
Groovy scales. The shark's scales are equipped with "microgrooves," aligned in the direction the animal is swimming. "These little microgrooves basically reduce the drag on the shark," Fish said.
One final advantage. Unlike Phelps, a shark does not have to come up for air.
"When you come to the surface, you have an extra form of drag on you, in the form of waves," Fish said.
The Discovery Channel revealed few details about the show in advance of broadcast. Phelps told Entertainment Weekly that the race distance was 100 meters in open water, though the two competitors were not side by side. Trained divers also were in the water to make sure all went well, and at one point Phelps got to see the sharks up close from the safety of a cage.
Once the Olympian understood what would be in store for him, he was game.