LACROIX-SAINT-OUEN, France (AFP) — Equipped with powerful magnets, history enthusiasts and environmentalists scour France’s rivers, pulling out bits of scrap metal, bikes, scooters and odd kitchen appliances.

Sometimes, too, they fish other more unexpected objects out of the water, such as unexploded munitions.

Magnet fishing has fast become a popular pastime, according to public safety officials, but French authorities have clamped down over fears that historical battle sites could still harbor active weapons.

As in other countries, participants in France tie a super magnet to a rope and drop it into waterways — partly for treasure hunting, partly for environmental reasons.

On the banks of the Oise river, in a town about 75 kilometers (47 miles) north of Paris, Owen Gressier, 20, and his three fellow magnet fishermen cast their neodymium magnets.

“We’ve been fishing here for a number of years, the bottom (of the river) is pretty clean,” Gressier, a forklift truck operator, says.

Driven by what he calls his passion for World War II and a quest to find medals, military gear and other historical objects, Gressier says that he also “quickly realized that it was possible to clean up the waterways.”

Faced with the hobby’s rising popularity in the last nearly two years, including under the Pont des Arts pedestrian bridge in central Paris, the authority has recently made police across France aware of the dangers.

In May, a man was seriously injured after pulling out a shell that emitted mustard gas in the Nord region, home to the town of Dunkirk.

Two young magnet fishers in the Somme at the end of July also hauled out a phosphorus grenade, which irritated their eyes, police said.

The pastime is now illegal in France without a permit issued by the state or landowners.

“We were not aware of the risks… until my son found a grenade,” says Helene Ledien, who lives in the Somme.