Not everyone believed it would succeed. The Guardian newspaper asked in October 2023: "Can Paris clean up the Seine in time for next year's Olympics?" For months before the 2024 Games, newspapers and satirists mocked the plan, with headlines warning about bacteria, heavy rain runoff, and the risk of an embarrassing "swim in sewage".

But when the Olympic triathlon and marathon swimming events took place as planned last year, without any health issues, the city felt vindicated.

Marion Lefevre, a 41-year-old nurse who swam on July 6 at the Bras Marie site, summed up what many felt. "I never thought I'd see this in my lifetime," she said, smiling as she dried her hair. "Swimming in the Seine used to be something my grandparents talked about. Now it's real again."

The Seine stretches about 13 to 14 kilometers through Paris, but for now, only these three carefully monitored zones are open. But the message is clear: Paris's mother river belongs to Parisians again.

As the sun sets, the queues are still long. Locals and visitors alike dive in, shout to friends on the dock, and wave at sightseeing boats drifting past. A hundred years ago, the city had no choice but to ban people from swimming here. Now, they're back, and they're not letting go.

Mohammad Arif Ullah contributed to this story.