Swedish corvette HMS Visby patrols the Stockholm Archipelago October 19 2014, searching for what the military says is a foreign threat in the waters.Sweden beefed up its military presence in the Stockholm archipelago on Saturday to scour its waters for "foreign underwater activity," a mobilization of Swedish ships, troops and helicopters unseen since the Cold War. REUTERS/Marko Savala/TT News Agency

This amateur photo made available on October 19, 2014 by the Swedish Defence Ministry shows an object (top C) in the sea near Stockholm. Sweden released the grainy photo of a mysterious vessel in Stockholm's archipelago, as the military hunted for a foreign submarine or divers in the country's biggest such mobilization since the Cold War. REUTERS/Swedish Defence Ministry/Handout/TT News Agency

Swedish minesweeper HMS Koster patrols the waters of the Stockholm archipelago, October 19, 2014. REUTERS/Marko Saavala/TT News Agency

Swedish Rear Admiral Anders Grenstad speaks during a news conference in the headquarters of the Swedish Armed Forces in Stockholm, October 19, 2014. REUTERS/Jonas Ekstromer/TT News Agency