The mine is a Swedish-made F31 which was produced in 1931, but it is currently unclear when it ended up in the sea.
"It's difficult to say exactly when it was used. We don't know if it would fire or if it was an exercise mine – it has been in the water for a long time and is coated with vegetation, so the markings can't be deciphered,” explained Mattias Robertson, communications officer at the Swedish Armed Forces HQ (HKV).
The mine, which has been temporarily secured so it doesn't float away, is to be detonated by military staff on Monday. It will be towed into deeper water then destroyed there.
Around 20 people living on the Långgarn island where the mine was found will be evacuated from their homes for the blast, police have announced. The risk of something going wrong is small however, according to Robertson:
"Dealing with unexploded bombs always involved some risk, but it is not unusual for the Armed Forces to do these kind of things. The people taking care of the mine are experienced."
The Armed Forces were notified about the mine on Sunday after a member of the general public discovered it washed up near the island.
Last year an entire village in central Sweden had to be evacuated while a bomb squad blew up a live 380-kilo shell that had been sitting in a garden for 86 years.