The long, thin piece of metal looked like a scaffolding pole when Trevor Penny saw it on the banks of an English river last November.

That would not have surprised Mr. Penny, who, while practicing his magnet fishing hobby, has unearthed household items, tools and other metal debris from the waterways near his Oxfordshire home. (Magnet fishing is pretty much what it sounds like: A strong magnet is attached to a rope, which is then tossed into a body of water.)

But his find that day was much more dramatic: a rusty Viking sword that had been there for more than 1,000 years.

The sword, found in the River Cherwell and identified by an archaeological group that tracks public finds, most likely dates to a period between 850 A.D. and 975 A.D. Mr. Penny said he handed it over to the Oxfordshire Museums Service this week, where it is expected to be put on display after restoration.

The discovery evokes a medieval era during which Vikings landed in the British Isles, raiding, plundering and trading their way through England and Scotland. Their incursions did not last, but their descendants still make up part of the British population today. In recent years, metal detectorists have helped unearth artifacts and treasure that had lain buried underground for centuries.

Detectorists in 2007 found a hoard of Viking treasure near North Yorkshire that experts called one of the most significant finds in Britain. Another hoard buried during that era was discovered in 2014 in Galloway, Scotland. But not everyone who stumbles across such discoveries has been immediately transparent: Two men in 2019 were convicted of theft and ordered to pay $1.5 million after they concealed the discovery of an Anglo Saxon hoard of treasure and sold some of the items.