The police were alerted to Borsöknasjön on Monday evening after a private person saw something that looked like a hand grenade, according to information to SVT Sörmland.
Police have cordoned off an area that is now being patrolled by guards.
In the afternoon, the police divers will examine the object in more detail and while waiting for it, the place will be cordoned off.
According to the police, there is no danger to the public.