Magnet fishing

Magnet fishing, also called magnetic fishing, is searching in outdoor waters for ferromagnetic objects available to pull with a strong neodymium magnet.
The hobby is a combination of environmentalism and treasure hunting. The magnets used are strong enough to remove large debris such as discarded bicycles, guns, safes, bombs, grenades, coins and car tire rims from bodies of water, but many who engage in the hobby are hoping to find rare and valuable items as well. 
It is thought magnet fishing was initially started by boaters using magnets to recover fallen keys from the water. 
The hobby has been adopted by celebrities such as English rugby player James Haskell.
Tools.
Magnet fishing is typically done with gloves, a strong neodynium magnet secured to a durable rope between , and sometimes a grappling hook as a supplement to the magnet.
Potential harms.
Some environmentalists worry that removing objects can stir up sediment that contains toxic materials, or disturb archaeological sites. Some magnet fishers have retrieved dangerous objects, including loaded guns, unexploded ordnance, and sharp pieces of metal.
Neodynium magnets are powerful and can interfere with pacemakers, posing a health risk; they can also damage other electronic devices. Fingers can get crushed between the magnet and a piece of metal, potentially causing serious bodily harm. Tetanus can also be a risk for those without an up-to-date tetanus vaccine.
Two men are known to have drowned while magnet fishing, but the cause is uncertain, and post-mortem toxicology tests showed both had recently used cannabis.
Magnet fishing and the law.
Depending on the jurisdiction, anything of value may belong to the local government, not the finder.
England and Wales.
Magnet fishing is subject to local regulations concerning outdoor waters. The Canal & River Trust, which owns most of the UK's canals, has bylaws prohibiting people from removing material from the canal and rivers it owns, so fishers may be subject to a £25 fine for magnet-fishing or removing any material from canal or inland navigation under the control of the Canal & River Trust in England or Wales, other than the Lee and Stort Navigation, Gloucester and Sharpness Canal, and River Severn Navigation. The Trust "expressly prohibit[s]" the practice, although it refrains from legal action against first-time offenders. In 2018, a child magnet-fished a sawn-off shotgun out of the Titford Canal in Oldbury, West Midlands.
Germany.
In Hamburg, magnet fishing without a permit is punishable by fine.
Belgium.
Amateur magnet-fishers in Belgium helped the police by recovering new evidence, specifically firearms and ammunition, related to the crimes of the Brabant killers.
In general, police urge those who find weapons or similar items to contact them.
United States.
In the US, there are no federal laws restricting metal fishing. Laws involving the restriction of metal fishing exist only on the state and district level, and most only ban it in certain places of historic significance or government owned land