A Swedish Navy surface ship spotted a periscope of an unknown submarine during military exercises in Swedish waters last year, local news site Dagens Nyheter reported.
The news caught international attention because it happened only half a year after the 2014 major submarine hunt in the Stockholm archipelago. The 2014 hunt did not manage to identify the submarine but most analysts suspected it to be of Russian origin.
As for the 2015 incident, sailors assigned to a surface ship spotted a submarine periscope but did not undertake any actions as they only later realized that no Swedish submarine was scheduled to take part in the exercise.
The spotting was classified as “probable submarine”, which is the second highest degree of certainty in the Swedish Navy.
Since the 2014 incident, the Swedish Government has started upgrading its anti-submarine warfare capability by investing in surveillance boats, radars and anti-submarine missiles. Additionally, two Göteborg-class corvettes are being modified for anti-submarine warfare.