Presenters of the BBC's Beechgrove Garden discovered a sub-machine gun hidden in an old mill pond during a restoration project.
The gun has been handed over to police
Filming of an episode of the television series was halted after the WWII weapon was found at the site in Forres, Moray.
The pond is being reclaimed as part of a school project.
Gardener Mike McGlashan said the gun made an "interesting change" from the more regular supermarket trolleys.
Mr McGlashan added: "It was pretty rusty but still in fairly good condition considering the time it must have spent at the bottom of the pond."
Filming was halted while the weapon was taken to the local police station, where firearms experts identified the weapon as a 1930s Schmeisser 9mm sub-machine gun.
Officers believe it was probably a wartime souvenir that had been discarded.
A spokesman for Grampian Police said: "Exactly how it got there we will never know, but it was probably thrown there by the owner who had no further use for it.
The discovery was made in a pond
"But the condition it was in rendered it incapable of being reactivated."
The weapon is to be examined by firearms experts before being given to a local museum.
The gun was discovered buried deep in the ground which used to form part of the Plasmon Pond, Forres.
The pond is being reclaimed by pupils from Forres Academy as part of an environmental improvement project.