On Friday morning, five NATO ships together with the Swedish ship HMS Sturkö arrived at Visby harbor. They are now preparing next week's mission to clear mines north of Gotland. No one knows exactly how many there are.
"It's the best kind of training for us, to find real mines. Then, when we encounter new dangers and new mines, we are ready," says Commander Pyotr Bartosewicz of the Polish ship Czernicki, which is leading the mission.
NATO has been destroying mines since 1973, when it began in the English Channel. But there are still a large number of unexploded mines left in, among other places, the Baltic Sea. However, no one knows exactly how many there are.
In the video you will hear more about mine clearance and the mines that remain in the Baltic Sea.