Follow our Olympics coverage in the lead-up to the Paris Games.

PARIS — It’s been quite the spring in Paris, with the city set to host the Olympic Games for the first time in 100 years.

Temporary stadiums are rising at the foot of the Eiffel Tower, in the plaza next to the Orangerie (home of the Monet murals), in the gardens of Versailles. Most people though will never see what may be the most important Olympic facility, the $1.5 billion underground tunnel and water tank that is supposed to make the Seine, the river that flows through the heart of the city, suitable for the triathlon and the marathon swim races and beyond.

Yes, you read that right — swimming in the Seine. The river that makes hearts melt, the site of countless marriage proposals, where for years, couples would “lock their love” by writing their names on a padlock, attaching it to the Pont des Arts and tossing the key into the water. It is also the river that only those who crave a baptism by murk, sewage, fecal refuse and various other detritus would think of heading for a dip, which has been illegal for roughly a century.

The organizers of the Paris Games tried this out with some test events last year, including a triathlon. Kirsten Kasper, a longtime triathlete who will make her Olympic debut in Paris, was there. She remembers standing on the starting dock, “looking up at the Eiffel Tower, and just smiling.”

The “looking up” part probably had something to do with that.

Men’s triathletes dive into the Seine last summer as part of the test for the 2024 Olympics. A $1.5 billion underground system is meant to help clean the polluted waters. (Bertrand Guay / AFP via Getty Images)

As for the smile, that jibes with what Lambis Konstantinidis, the director of planning and coordination for the Paris Games, heard when he asked athletes about their time in the river.

“There was not one that did not say it was not a unique experience,” he said.

That is one way to describe it.

Whether any of the Olympians and Paralympians preparing to compete in the Seine get the chance to swim in the river remains an open question. It turns out that a $1.5 billion water tank intended to catch sewage during rainstorms that would normally flow into the river — plus years of forcing houseboats, ships and factories to stop polluting the river — can only do so much.

Officials inaugurated the Austerlitz water basin, which is located underneath the Austerlitz train station on the river’s Left Bank in the southeast quadrant of the city, in early May. It can hold 13.2 million gallons of water — enough to fill 20 Olympic-sized swimming pools.

In late May, rain fell on Paris for a week. That wreaked havoc with play at the French Open and rendered the Seine unswimmable because the rain overwhelmed the tank and tunnel system, and street runoff and fecal matter flowed into the river once more.

Officials knew this could happen. They know it might happen during the Olympic Games, though late July and early August, when the Games will take place, are generally warm and dry in the French capital. They hope weather patterns hold.

World Aquatics, the world governing body for swimming, recommends that organizers of open water events consider alternative locations to manage a drop in water quality on race day. Paris officials considered their options, but ultimately decided to hope it doesn’t rain, and that the warm sun of a typical Paris summer can kill enough of the dangerous bacteria.

There is no Plan B, other than postponing races for a few days to let the yucky water flow downstream. They say they could also turn the triathlon into a duathlon, comprised only of cycling and running, but there’s no pristine lake on the city’s outskirts on standby for the 6.2-mile swim race.

“Nothing will be done to put the athletes at risk,” Konstantinidis said.

Paris organizers are counting on a newly constructed water basin beneath the Austerlitz train station to keep the Seine clean during the Olympics and beyond. (Christine Poujoulat / AFP via Getty Images)

Whether the water will be clean enough for competition has become a quadrennial conversation for Olympic organizers who have increasingly leaned toward locating these events in scenic waters that look great on television. Racing in open water isn’t all swimming off the coast of Kona, Hawaii, at the Ironman World Championships. But the tradeoff for beautiful sights on television and competitions in the heart of the cities that host them is often water that is kind of gross.

In 2016, Rio wanted to put the swimmers off the beaches of Copacabana, which for years have been the receptacles for the city’s sewage. Five years later, Tokyo had the swimmers compete in Odaiba Marine Park in the city’s busy harbor, which also harbors plenty of the city’s sewage and runoff. Officials installed a series of screens that were supposed to catch some of the harmful bacteria from the excess flow.

Morgan Pearson, a favorite to medal in triathlon for the U.S., said the water in Tokyo was “much murkier” than what he experienced at the test event last year in Paris. He skipped a practice swim in the river because he figured getting more familiar with the current wasn’t worth the risk of possibly getting sick.

“I’ve been in cleaner water in my life,” Pearson said of the Seine, “but there wasn’t anything that stuck out.”

Indeed, bacteria rarely does.

Like all organizers of major open water competitions, the people in charge of the Paris Games will comply with the World Aquatics standards for safe swimming set by the World Health Organization for the levels of bacteria most closely associated with sewage contamination — E. coli and enterococci.

The open-water venue will certainly pop on TV, but health concerns for athletes swimming in the Seine will persist through the Olympic races. (Bertrand Guay / AFP via Getty Images)

That requires a classification of “good water quality” which, for those microbiology majors out there, means less than 500 “colony-forming units” of E. coli per 100 milliliters of water and less than 200 units of enterococci. A colony-forming unit is a collection of cells. The Seine will also have to pass an eye test for murkiness and floating debris. The tests are supposed to take place several days ahead of the competitions and at multiple locations along the course.

Taylor Spivey, another member of the American triathlon team, grew up lifeguarding on the beaches of southern California near Los Angeles. She knew from an early age that swimming after a rainstorm was a bad idea. She has not forgotten it. She swam in the Seine last year during the test event.

“No one got sick,” she said with a smile.

The prayer of all Olympic organizers is that the Games leave a legacy and change their cities. For the French, making sure the competitors in the Olympics and Paralympics are not the last ones to swim in the Seine is a major part of that.

There are canals in the city that already allow limited swimming. The city plans to open three swimming areas along the river in 2025, assuming the Austerlitz water basin can do its job and the city’s residents are ready to take this very specific leap of faith.

“Parisians are getting used to the idea” of swimming in the urban waterways Konstantinidis said, “but they will need to see it.”

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(Top illustration: Dan Goldfarb / The Athletic; photo: Tim Clayton / Corbis via Getty Images)