At 20 seconds past 8 a.m. here Wednesday, 55 female triathletes plunged into the Seine just below the Pont Alexandre III. The Nymphs of the Seine carved into the bridge’s arch, a fleet of photographers and a smattering of spectators on the river’s banks were there to witness it.The splashes barely could be heard over the cheers of fans gathered in bleachers along the riverbank and on the streets several feet away. But it was a massive leap for a city that longed to use the Paris Olympics to make the river safe for swimming again.

For nearly 100 years, the Seine has been considered unsafe for swimming because of massive pollution, and political leaders from Paris and surrounding municipalities have spent 1.4 billion euros on a sprawling cleanup project that was supposed to be finished in time for Olympic triathlons and marathon swimming events.