As reported by the StarTribune:
Long before next week's fishing opener, a few Minnesota anglers were avidly casting lines into the water and hauling in hefty catches. But they weren't hooking walleye, bass or northern pike. They were hoping to reel in an antique metal sign, a safe full of money, maybe even a gun.It's part of a new, and admittedly niche, sport of magnet fishing, where you "fish" with a super-powerful magnet tied to a strong line. Most of the time, magnet fishers pull in mundane scrap metal: old nails and bolts, a length of rebar, fish hooks, beer bottle caps...Even if it's just scrap metal, local magnet fishers don't catch and release, but rather dispose or recycle their finds and consider themselves to be helping the environment by hauling hundreds of pounds of metal out of local rivers and lakes...The hobby is also growing in parts of Europe, where there's a lot of metal in canals and other bodies of water thanks to centuries of warfare."England is very, very big for magnet fishing," Shoemaker said.
More information at the link. I've wanted to do something like this ever since I was a kid in a boat spotting lost lures at the bottom of a lake.