Historical enthusiasts and environmental activists equipped with powerful fishing magnet clean French rivers by collecting them with scrap metal, bicycles, scooters and personal kitchen equipment. Sometimes they catch other unexpected things from the water, such as unexplained weapons.
According to public security officials, magnet hunting is fast becoming a popular pastime, but French officials are still concerned that historic war sites are still carrying active weapons.
As in other countries, participants in France tie a rope to a supermagnet and release it into the waterway, partly in search of treasure, partly for environmental reasons.
Neodymium's magnet, 20-year-old Owens Grasier, and three of his colleagues magnetically threw a magnet on the banks of the River Oise in a city 75 kilometers north of Paris.
After several attempts at his place near the La Crixa-Saint-Ouen bridge, he caught something, a few minutes with a tangible hook pull. As they say, a rusty iron tube shows: "Good grip". It was the best discovery that afternoon.
"We've been fishing here for many years, the (river) bed is very clean," says truck operator Grayser.
Inspired by a passion for World War II memorabilia and the discovery of medals, military equipment and other historical items, Mr Gracis says he "quickly realized that it was possible to clear the waterway."
In 2017, he created a Facebook page that now has more than 500 subscribers, where members share photos, tips and organize trips. "It's crazy to find everything in the water," Mr Gresier lists everything from electric scooters to traffic lights and microwaves.
"With dozens of people, sometimes in a matter of hours you can pull out 50kg of scrap metal," he says.
In one of the greatest wars of the First World War, Christoph Devarin started fishing for magnets in the neighboring province of Somme three months ago.
He says "the secret doesn't know what will happen to the magnet."
But the 52-year-old driver warns that "if you're thinking of finding treasure, it's not much."
Although he pulled out a rifle dating from 1914 to 1918, he says it "is completely rusted in the water after 100 years."
"Even the Museum of the Great War didn't want that," he joked that there was no sign of waste because it was given to resellers or scrap dealers.
Even in other regions of France, where bloody battles took place during the world wars, magnets discovered shells, ammunition and grenades.
They may still be active, warns the National Public Security Administration.
Given the hobby's growing popularity over the past two years, including under the Pont de Arts pedestrian bridge in central Paris, the authority has recently revealed dangers to police throughout France. In May, the man was seriously injured after the withdrawal of a mustard gas bullet north of Dunkirk.
Police said that at the end of July, Somma's two fishermen with a magnet seized a phosphorus bomb, causing them jealousy.
Entertainment is now illegal in France without a permit issued by the state or zamindar.
"We didn't know about the danger ... until my son found a grenade," says Helen Leiden, who lives in Somme.
She said her 14-year-old son Arthur bought a magnet from Amazon for about 30 euros ($ 33) and regularly fishes with a friend for environmental reasons.
Greser said his group captured active projectiles and hundreds of rusty weapons, but knew what to do in the case.
"We are creating a security perimeter and warning destruction specialists," he says.
Despite warnings, his group says it will not stop the activities they find.
His friend Nicholas, 26, says, "It's a passion, good for the planet, we won't stop at night."
"People are going to play cat and mouse," laughs Devarins.