The observations were made by youngsters at a sailing camp at the Bosön peninsula north of Lidingö island – north-east of the Swedish capital's city centre – in the morning of June 28th.
The children and their teenage instructors first saw large air bubbles in the water and then a large, black object near the surface, writes DN. Three instructors then saw what they perceived to be a large submarine close to the shore. DN has published the group's mobile phone video of the vessel.
— Dagens Nyheter (@dagensnyheter) October 25, 2018
It was visible for around 20 minutes as it continued sailing in an eastbound direction.
The group did not immediately report the incident, but it came to the attention of the Swedish Armed Forces a few days later via word-of-mouth, and they sent out two officers to investigate the claims.
“We can confirm that we have received information about an alleged observation in this area at that time,” press spokesperson Jesper Tengroth told DN. He also confirmed that they had spoken to one of the witnesses, but declined to elaborate, saying that the information was “confidential”.
“We will not comment on any further actions that we have taken or will take. We neither comment on results nor our view of alleged sightings,” he told the newspaper, which also interviewed the witnesses.
DN writes that no known Swedish military or civilian submarines were in the area at the time.
The newspaper ordered its own marine survey of the area ten weeks after the vessel was sighted, which showed no evidence of any underwater activity. However, Nils-Ove Jansson, a now-retired former deputy head of Swedish military intelligence, told DN after seeing the video that on a scale from 1-5 where 1 is “confirmed submarine” and 5 “not a submarine” he would place it at 2, “probable submarine”.