Wiltshire Police are warning magnet fishing devotees of the potential dangers
A spokesperson said: "Fuelled by many videos on social media, the hobby appeals to people of all ages, who use a strong magnet to hunt for metal objects submerged in canals and rivers.
"They are recovering everything from scrap metal and jewellery to firearms and hand grenades.
"The danger comes when finding old ordinance, such as hand grenades, mortars or other unexploded devices. Depending on how long they have been submerged they can be highly unstable and may explode when disturbed or by handling when out of the water."
In the past year both Trowbridge and Salisbury have had streets closed following the discovery of a large artillery shell and WWII hand grenade respectively, resulting in the closure of roads until they could be safely dealt with by the bomb squad.
Although there are no specific laws against magnet fishing, participants could be committing offences under the Theft Act if they fail to seek permission from the landowner.
The spokesperson added: "Everywhere and every time you use the magnet to fish, you will require permission of the owner of the land you enter to access a river or canal.
"These waterways may also be under the control of another authority such as the Canals and River Trust, who prohibit any magnet fishing in the canals of England.
"You need to get clear written permission and carry that with you so you can produce it when asked to do so.
"Should you locate items that could be considered as Treasure under the Treasure Act 1996, you have a legal obligation to report items to the local Coroner within 14 days of making such a find."
There is more information about treasure on the Portable Antiquities web site at www.finds.org.uk