Paris isn’t Paris without the River Seine.

The 780-kilometer-long river, which runs through the city of light and curves around the mighty Eiffel Tower, not only hosts tourists taking in the sights of the French capital but also supports centuries-old UNESCO-protected French bookstalls on its banks. It is an indispensable part of the Parisian experience, just like the Thames River is to London.

Despite being considered the most romantic river in the world by virtue of its location, that’s not the first thought crossing people’s minds as they peer into Seine’s murky, sometimes stinky waters.

In the past few months, the Seine has been gearing up to take on another role: a controversial center stage for the 2024 Paris Olympics, which will kick off later this month.

French authorities sought to put Seine in the spotlight at the global event, which is as much an opportunity for theatrics and soft power as it is a platform for sporting talent.

Paris announced plans to clean up the river and open it up for swimming for the first time in 100 years (even though Parisians dipped into it a few times after). More “firsts” followed—in an uncommon move, the Olympics organizers said the Games’ opening ceremony would be held outside the stadium and on the Seine instead, sparking security concerns. President Emmanuel Macron joined the fanfare by vowing to swim in the river to prove its safety.

He isn’t the first president to do so—in 1990, Paris mayor-turned-President Jacques Chirac also said he’d clean up the river and take a dip in it.

But apart from being an ambitious stunt, the Seine’s clean-up reflects the present-day challenges associated with such mammoth-sized efforts, particularly if the undertaking has historical importance. Paris is hosting the Olympics for the first time in 100 years. A Frenchman, Pierre de Coubertin, was behind reviving the Games in the late 19th century, making the event even more significant for its hosts.

Amid all the hopes vested in the Seine, it’s had a rough couple of months. With just weeks to go before the Games start and $1.5 billion already spent on clean-up efforts, the river’s readiness for swimming still remains in question. After a series of failed tests, the Seine’s water quality has begun improving, the Paris mayor’s office announced on Thursday.

“This positive development is a consequence of the return of sunshine and warmth as well as the effects of the work done as part of the strategy to improve the quality of the Seine’s waters,” the office said in a statement. However, given the delicate link between wet weather and the Seine’s contamination levels, things can still change before the Games commence.

The Seine turned into a protest site in June when activists threatened to defecate the river for the same reason, reflecting how the river is a crucial but sensitive part of the Olympics. But without the push of the Olympics, perhaps there wouldn’t have been much of an incentive to make headway with cleaning such an iconic river, Lindsay Krasnoff, a historian and global sports expert, told Fortune.

“The Seine plays this dualistic role between our Emily in Paris postcard moments and the reality of being a living, working, breathing river,” Krasnoff said.

Last month, E. coli levels exceeded the safe threshold in a test by water monitoring group Eau de Paris. Ironically, those results were announced just after the International Olympic Committee’s executive, Christophe Dubi, said he was “confident that we will swim in the Seine this summer” and had “no reasons to doubt” that the Olympics will progress as planned.

The river is both the backdrop and the main character in the Paris Olympics and how it emerges from them could offer lessons for future hosts of the Games.

Undertaking to clean the Seine up

Paris’s efforts to clean up the Seine aren’t new. The result of the decades-long attempt can be seen in the thriving fish species it houses, up from three in the 1970s to about 35 now.

But reversing years of waste discharged into the river was never going to be easy. The plumbing system in many Parisian homes still has direct outlets to the river instead of wastewater drainage pipes, and homeowners will need to pay out of their pockets to change that.

The impact of climate change also plays a significant role in how the Seine adjusts to its clean-up efforts, said Jay Famiglietti, a professor at Arizona State University specializing in sustainability and global water risk. Heavy rainfall can overwhelm the sewer system, resulting in untreated water entering the Seine.

“Today, these events are occurring with more frequency,” Famiglietti told Fortune. “If there’s one thing that perhaps slipped under the radar in the planning [of the Paris Olympics], it might have been accounting for the fact that the frequency of these intense storms is increasing.”

In recent months, authorities have tried various measures to clean up the Seine, including constructing a storage basin that prevents water from entering the river and degrading its quality.

Even though Parisian authorities have been working on the effort for years, it can be tricky to anticipate the forces of climate change and their impact on such colossal undertakings as cleaning the Seine.

“Do we have a 100 percent guarantee? The answer is no,” Pierre Rabadan, Paris’s deputy mayor overseeing the Olympic plans, told the New York Times in May. “If it rains for a week continually before the races, we know the quality of water — even with all the work that has been done — probably won’t be excellent.”

Representatives at the Paris Olympics 2024 organizing committee didn’t return Fortune‘s request for comment.

A glimpse of the Bassin d’Austerlitz, a huge water tank which aims to clean up the Seine River.

Looking to the future

Weather changes aside, Parisians aren’t sure they’d dip in the Seine—less so when its water keeps failing contamination tests. It’s still unclear what might happen to some of the Olympic events if the river water doesn’t pass the safety standards. France’s Sports Minister Amelie Oudea-Castera said earlier this year that “there is no plan B” because some form of plan A involving cleaning up the river will just have to work.

To be sure, projects like the Seine’s can be a hit or a miss. Large-scale events have prompted the beautification drives of urban landscapes in the past, as in the case of Shanghai Houtan Park created ahead of the Shanghai 2010 World Expo, Jennifer Minner, a Cornell University professor specializing in the impact of mass events on their host cities, pointed out.

But there are also cases of spectacular failures—Rio de Janeiro infamously failed to clean up Guanabara Bay despite lofty promises to do so ahead of the 2016 Olympics. While the jury is still out on the Seine, Paris still sets an example for future Games.

“Mega-events such as the Olympics and World Expos can be used as catalysts or accelerators for all sorts of urban transition. They spur imagination about how host cities and regions can grow and change,” Minner told Fortune. She said that by elevating the role of urban ecology, Paris could set a precedent for future Games.

There’s also something to be learned about river clean-up efforts from Paris’s case: it will take much more than scrambling in the few years before the deadline.

“We’ve seen time and time again that rivers can be restored.. it just takes a long-term commitment. It can’t be a political flip-flop because it will never happen,” Famiglietti said.

Paris still plans to open the Seine to the public next year. However, with a possible change in government and the sports event cycle closing in August, it’s unclear if cleaning the Seine will be a top priority for French leaders.

The Seine project puts into perspective what being a host city for a grand event means, aside from the glitz and glamor. Paris set out to make its iteration of the Olympics the most sustainable version of the Games yet by minimizing new venues and recycling water used for various sporting events. Los Angeles plans to double down on such efforts when the host baton is passed on to them in 2028.

“The desire to also help to reset what it means and what it takes to host the Olympic and Paralympic cycles given the environmental and sustainability issues … that’s just the tip of the iceberg,” Krasnoff said.