Higher, stronger…slower? Why the Paris Olympic pool has a problem

The pool constructed on the customary site of a rugby pitch measures 2.2 meters deep, or roughly 7-feet-2, which means French basketball star Victor Wembanyama could comfortably stand in it. At recent Olympics, the pool has always been around 3 meters deep—a difference of nearly 3 feet.

It might sound strange to anyone who lacks the amphibious qualities of an elite swimmer to hear that such a small amount could possibly matter. But in a sport where contests are settled by as little as a hundredth of a second, even the tiniest variation in pool specs can determine whether records fall—typically a hallmark of Olympic swimming.

So far, the world records have stayed stubbornly intact, even those that seemed primed to crumble due to the collection of talent gathered here. Not even swimming’s biggest stars, like France’s Léon Marchand and Australia’s Ariarne Titmus, have managed to conquer the waters of Paris La Défense Arena. But it turns out it might not be the athletes who are to blame—but the pool itself.

“The Olympics are all about racing, it’s all about getting your hand to the wall," said seven-time gold medalist Katie Ledecky. The problem for the Paris Games is those hands are touching the wall a little slower than normal.

Paris 2024 organizers didn’t respond to a request for comment.

With capacity for 40,000 spectators—massive for an indoor arena—La Défense Arena has a cavernous interior. That’s perfect for its usual tenants. French rugby club Racing 92 calls the arena home. Taylor Swift recently had a four-night stand here.

But what made the venue ideal for the Eras Tour presented an issue when trying to convert it into a natatorium.

“To go deeper, they would have had to do significant modifications, and structurally, there are some concerns," said John Ireland, the chief technical officer at Myrtha Pools, the Italian company that designed the pool for the last five Olympics.

To understand why that matters, it’s important to remember that swimming isn’t just a dynamic exercise. It’s also an exercise in fluid dynamics. Every movement swimmers make with their hands and feet create small waves that rebound off the bottom of the pool.

No movement has more potential for waves than swimmers’ initial dive into the water. The shallower the pool, the more those waves ricochet back to the swimmers on the surface, creating a more turbulent aquatic environment that isn’t conducive to top speeds.

In the future, pools as shallow as the one at these Games won’t be allowed. World Aquatics enacted a minimum depth of 2.5 meters for swimming and water polo in January 2023. When Paris had its bid accepted for the Olympics in 2017 and the swimming facility was approved, regulations allowed for a pool of 2.2 meters. The pool that will be used for artistic swimming, which is located in a newly constructed building in nearby Seine-Saint-Denis, is three meters, a World Aquatics spokesman said.

Jud Ready, a Georgia Tech engineering professor, said the effect of the pool depth is particularly pronounced at the end of laps, as the flip turn causes large disruptions to the water.

“Certainly the shallower it is, the more energy that is returned," Ready said.

Ditto for the dive. In shallow pools, swimmers that jump out from the blocks farthest may earn a minuscule advantage—their entry will put them ahead of most of the waves.

There may be a psychological component at work, too. Ready said he would expect swimmers to feel as if they’re swimming faster than they actually are in a shallower pool, as they are accustomed to seeing the bottom zip by from 3 meters up—but in reality, they’re barely 2 meters above the ground.

He compared this phenomenon to being on an airplane and how it seems fastest when it’s closest to the ground. After claiming a bronze medal in a race Saturday, Ledecky said her swim “felt a little faster" than the stopwatch ultimately indicated.

Ireland pushed back against the notion that the pool his company designed has slowed down swimmers. He insisted that while the difference between a 1-meter and 2-meter pool is considerable, the difference between a 2-meter and 3-meter pool is much less dramatic.

“The international standard has been created because there is a negligible impact to swimmers’ performance after 2 meters," Ireland said. “There’s also a perception that deeper is better. They’re not wrong, but there are other factors."

Those other factors, he said, include the temperature of the water, the air quality in the venue and the design of the gutters at the edges of the pool.

Whether it’s just one of those elements or a combination, the results from the Olympic pool suggest that something is definitely up. Times in many of the races have been slower than expected, and while a few new Olympic records have been set, no world records had fallen through the first two days of swimming.

That includes perhaps the signature race of the entire Games: the women’s 400-meter freestyle final, a much-anticipated showdown between Titmus, of Australia, Ledecky, and Canadian phenom Summer McIntosh. All three have held the world record at various times in the last two years, and it seemed certain it would take another to claim gold.

It didn’t happen. Titmus won, but was a full two seconds slower than she swam at Australia’s Olympic trials last month. Ledecky didn’t even manage to break 4 minutes—her time of 4:00.86 in the 400 was the 40th fastest of her career.

Afterward, Ledecky said that she, Titmus and McIntosh “probably all would say we would have liked to have been a little faster." Titmus agreed, adding that her finish was “probably not the time I thought I was capable of."

Two world-record holders swam on Sunday as well: Marchand in the 400-meter individual medley and American Gretchen Walsh in the 100-meter butterfly. Neither managed to set a new mark.

Marchand had to settle for something else: a gold medal.

“My objective was to try to beat the record this evening," said Marchand, who finished about a half-second slower than his personal best. “I felt as though I was in shape."

Though he missed out on goal, he found solace knowing that when he got his hand on the wall, he’d be an Olympic champion. “That was a great moment."

Write to Jared Diamond at jared.diamond@wsj.com and Laine Higgins at laine.higgins@wsj.com