On a hot July afternoon, Arthur Flynn III was on a kayak when he dropped a large magnet attached to a rope into the murk of the Nashua River.
The 60-year-old from Ayer was floating near Fort Devens when he pulled up an unexpected catch: an MK-2 “pineapple” grenade and a 60mm mortar shell.
He was soon met by State Police and a local bomb squad.
“The subject in question was advised to discontinue his endeavors,” a trooper wrote in a police report, noting that the weapons had been “severely corroded."
Flynn’s haul was the result of a peculiar but increasingly popular pastime known as magnet fishing, which mixes an environmental impulse to remove trash with a zeal for seeking sunken treasure.
But the benefits of removing a range of refuse from rivers, lakes, and other waterways comes with dangers, such as hoisting unexploded ordnance and discarded weapons, or disturbing contaminated sediment and submerged archeological artifacts.
The idiosyncratic hobby appears to have gotten a boost from the pandemic, which has left many people with less to do and more time to watch online videos that show everything from safes to assault rifles being hauled up from the deep.
Downtime with his bored 10-year-old grandson was what led Douglas Carvalho this summer to spend about $200 on ropes, carabiners, special gloves, and large magnets, including one that can lift as much as 760 pounds.
With their new equipment and a cooler packed with snacks, the two have been going to nearby rivers and the local marina, where they attach their ropes to the powerful magnets, drop them in the water, and scour the bottom, waiting for a tug from something metallic.
They found their new hobby was a lot like actual fishing, but with more intrigue.
“It’s kind of relaxing, even if you don’t get anything,” said Carvalho, 54, who lives in Fall River. “There’s the thrill of the unknown.”
They haven’t found anything as exciting as what they saw on YouTube, but they’ve brought up old railroad spikes, fishing lures, and a bolt hinge.
Passersby have asked if they were searching for gold, Carvalho said.
“No, I tell them, gold isn’t magnetic,” he said.
While Carvalho and other magnet anglers said they recognize the potential dangers, they insist it’s largely safe and would prefer that the state not regulate their new hobby.
But after Flynn’s experience near Fort Devens, state and federal officials, along with some environmental advocates, are raising alarms and calling for regulation.
The discovery of the unexploded munitions led officials at the Environmental Protection Agency to chastise the Army, which still oversees the shuttered military base, for not alerting the public more widely to the discovery of the old weapons. A few weeks after the grenade and mortar shell were pulled to the surface, the EPA noted, someone else magnet fishing near the base pulled up what appeared to be another explosive device.
In a letter last month, Carol Keating, an EPA official in Boston, told the Army she was “extremely disappointed” by its “continued noncompliance with its responsibilities.”
She urged the Army to investigate the extent of the problem and said the likelihood of other unexploded munitions in the river poses “an imminent threat and substantial endangerment to human health or the environment.”
As weeks passed without a response, the EPA invoked a rarely used dispute-resolution process between government agencies to press the Army to take action.
Since then, Army officials have started a “removal site evaluation” — an assessment of the remaining ordnance and the viability of retrieving it — which they said would continue until December.
“The Army reminds the public that keeping munitions, even Civil War cannonballs, as souvenirs is dangerous,” said Robert J. Simeone, an environmental coordinator for the Army overseeing Fort Devens.
He also noted that MassDevelopment, a quasi-public state agency overseeing the redevelopment of Fort Devens, issued an emergency order last month banning magnet fishing on a portion of the Nashua River and posted warning signs in the area.
The agency has also scheduled a public hearing to discuss a permanent ban on magnet fishing near the base.
Besides the dangers of finding weapons — someone magnet fishing in Jamaica Plain this summer pulled up a loaded snub nose revolver — state environmental officials said the hobby could also threaten archeological sites and removing certain artifacts from such areas may be illegal.
“Artifacts [of archeological significance] recovered from all Massachusetts' submerged lands — inland as well as coastal — belong to the commonwealth,” said Craig Gilvarg, a spokesman for the state Executive Office of Energy and Environmental Affairs.
Environmental advocates said they had mixed feelings about magnet fishing, which apparently began with boaters using magnets to search the abyss for missing keys.
“While removing trash from a river is generally a good thing, in some cases, stirring up sediments to get something that’s deeply wedged in the riverbed could be harmful for the aquatic ecosystem,” said Julia Blatt, executive director of the Massachusetts Rivers Alliance. “I’d certainly want to see the state set some guidelines around it to make sure no rivers are harmed.”
For Josh Parker, who took up the hobby after his wife and kids bought him a magnet fishing kit on Father’s Day, it’s a public good.
Given that the chances of someone pulling up a firearm or something explosive are small, magnet fishing shouldn’t be regulated, he said.
“It’s good-minded people cleaning up rivers,” said Parker, 34, an animal control officer from Brockton.
He takes his children to the Taunton and Charles rivers. Using magnets that can lift as much as 1,700 pounds, he has pulled up bicycles, shopping carts, and rusty tools. He even pulled up a portion of a safe, but it lacked any loot, he said.
He’s earned a few dollars selling metal he has found to scrap yards, but his motives are mainly environmental, he said.
“Unlike fishing, we’re not going out looking for something to eat, or a trophy item,” he said. “The main point for me is cleaning the river. It’s like picking up trash along the road.''
For his friend, Sean Martell, there are other motives. Among them: building an online audience for his growing YouTube Channel, where he posts videos of his spoils.
Since he took up magnet fishing after the pandemic hit — when he lost jobs repairing cooking equipment — Martell has hauled in the barrel of a sawed-off shotgun, a stop sign, and a washing machine.
He gave up fishing, he said, after a sea gull flew off with a mackerel he caught. Now, he only worries about losing magnets.
Most recently, the 44-year-old from Brockton latched onto something so big he couldn’t pull it up himself: a Ducati motorcycle. He called police, believing it was stolen, and they called in heavier equipment to remove it from the Taunton River.
“I was just in shock,” he said. “It’s a surprise every time you pull something out of the water. It could be good things, or trash, but it’s all about the find.”