Guns, shopping carts and bottle caps, lots of bottle caps: these are among the discoveries made this summer by people who have taken up magnet fishing, a hobby that works exactly as its name suggests.

Magnet fishing became popular during the Covid-19 pandemic, and, this summer, enthusiasts have been fined for pulling rockets out of a river on a military base in Georgia and been given an award for removing more than 20,000 pounds of metal from a river in Maine.

YouTube and TikTok are filled with videos of people using high-powered magnets attached to synthetic rope to haul vintage bicycles and antique firearms from water, but Ben Demchak, who has been magnet fishing for five or six years, said the most common finds are more mundane.

“What you’re not really seeing is all the scrap in between,” said Mr. Demchak, who in August 2020 founded Kratos Magnetics, a company that sells magnets and magnet fishing kits.

He was immediately flooded with orders and emails from people who were intrigued by a hobby that would take them outdoors, is inexpensive — the most basic kits can be found online for $20 — and is easy to start. He said he had not been able to keep up with the demand for magnets, even the most basic of which are stronger than a refrigerator magnet.

Mr. Demchak, an archaeologist, became interested in the hobby after he stumbled across a magnet fishing video on YouTube uploaded by someone who had found a gun.

“It’s a little different from the historical stuff I find,” he said. “It was the thrill of the hunt and the mystery of how it got there.”

In the United States, where there are more guns than people, it is not uncommon to find a firearm lurking in a local river or stream. People have also found knives, grenades and ammunition.

In late June, a magnet fisherman with 410,000 YouTube subscribers and two people who were with him were fined after finding 86 rockets and ammunition in a river at Fort Stewart, an Army base in Georgia where magnet fishing is banned. In July, two friends pulled an unexploded military shell out of the Passaic River in northern New Jersey. In May 2021, a man found a grenade in the Clarks River in Kentucky.

Magnet fishers are more likely to find scrap metal, which some enthusiasts sell for cash. Mr. Demchak said the most important thing is that magnet fishers are cleaning up waterways.

“People have been throwing stuff in the water for ages, ages,” he said. “I just pulled up an electric scooter, so it’s good to get that out of the water.”

Angel Lynn Carbone, who started magnet fishing a year ago and documents her experiences on TikTok, said people have started swimming in a lake near her home again because the lake bed is no longer covered with fishing hooks. She estimated that she pulled about 1,000 hooks out of the water in July alone.

Ms. Carbone, 50, who lives in Noblesville, Ind., said her hobby was like therapy.

“I am a person whose mind races, like every thought in the world,” Ms. Carbone said, speaking by phone from her garage, which she noted had 400 to 500 pounds of metal in it. “When I go magnet fishing and I throw that magnet in, the only thing I am thinking about is a plethora of things that could come up out of the water.”

Ms. Carbone said her most treasured find was a flashlight she was able to restore and put to use again. Overall, she estimates that she has pulled up more than 6,000 pounds of metal from waterways in about five states. Each month, a man comes by to collect most of the finds for recycling.

“It serves a purpose because, unfortunately, the waterways have gotten dirty and in my mind it doesn’t really matter how the waterways got dirty,” she said. “What matters is there is actually something I can do to help.”

Timothy Hoellein, an associate professor at Loyola University Chicago, also spends a lot of time cleaning up bodies of water because he is an aquatic ecologist who studies pollution. He says there is no way of knowing exactly how much garbage is in the nation’s ponds, rivers and streams, but it is a problem likely as old as human history.

People have long used rivers to get rid of things, he said.

“They were dumping grounds for food waste and other waste and it kind of goes away, it disappears, it will move downstream,” he said.

In his work, which currently involves removing garbage from urban streams and rivers in Chicago, Toronto and Massachusetts, he mostly finds trash related to single-use consumption, such as plastic bags, plastic bottles and aluminum cans. He said he has found a bow and arrow and bullets, but no guns.

Plastic waste is a bigger concern than metal waste because it has a greater biological impact on humans and wildlife. Floating plastics are consumed by wildlife, can entangle or trap organisms, contain potentially toxic chemicals and can absorb additional pollutants as they drift.

Though removing metals from bodies of water might not be the most pressing need for the environment, Professor Hoellein said he supported activities, including magnet fishing, that help people become more connected to their local rivers.

“These are bodies of water that are so long neglected but have so much potential to offer,” he said. “And if we change our system of valuing these ecosystems and think of them as an asset rather than as a place to dump trash, maybe they can be a real place to build community, well-being and education.”

Casey Deyoe, 32, has always had a close connection with water. She grew up in Mississippi, where her father was a river boat captain, and was a free diver before she took up magnet fishing a year ago.

The magnet fishing waters she explores in Northern California are murkier, and often smellier, than the clear waters she once dived in, but she said her new hobby has strengthened her relationship with the water. Crucially, it has allowed her to share that relationship with others through her YouTube videos and in conversations with curious strangers.

Ms. Deyoe said that when she goes magnet fishing, at least 10 to 15 people ask her about what she is doing. She often encourages them to try it for themselves, handing over her equipment. The last time she made that offer, the man she was speaking with insisted he couldn’t do it.

“The next thing I know,” she said, “I gave him a magnet and then he’s pulling up other stuff too.”