Gareth was fishing in North London when he made the shocking find.
Magnet fishing is a pretty unusual hobby but it's one you might be interested in taking up once you hear this guy's story.
Gareth Bryer, 36, from Manchester, took up magnet fishing at the start of this year and documents his finds on his Youtube channel. Using a powerful neodymium magnet he's caught some pretty interesting stuff including cash boxes, safes, a crossbow, a samurai sword and a knuckle duster but one find in particular shocked him more than any other.
In July, Gareth was fishing in a canal in Enfield, North London, when he pulled a submachine gun out of the water. Stunned at finding a potential murder weapon, Gareth phoned the police on a non-emergency number to let them know before returning to fishing - when he found the gun's magazine with a knife attached to it!
Armed police later met up with Gareth to collect the weapons and Gareth took to Youtube to share the incredible footage - which you can see in the video above!