Swedish minesweeper HMS Kullen is engaged in a military operation in the Stockholm Archipelago, Sweden, on Oct. 19, 2014.  (Marko Saavala/EPA)

If the reports are true, somewhere beneath the waters near Sweden's eastern coast lies a submarine. The submarine is about 65 feet long and 11 feet wide. It has no signs of damage and the hatches are closed: Whoever was aboard the submarine when it went down was unlikely to have escaped, Ocean X Team, the group who claimed to have found the submarine, announced.

Perhaps the most tantalizing clue, however, was the cyrillic language on the side. It appeared to be proof that the submarine originated in Russia. After Swedish newspaper Expressen published video of the submarine Monday, that detail provoked considerable, and understandable, attention.

Throughout the Cold War, officially neutral Sweden was harassed by Soviet submarines, most notoriously during the 1981 "Whiskey on the Rocks" incident, when a Soviet Whiskey-class submarine ran aground on the south coast of Sweden. Just last year, there were signs that Russia had returned to old tactics: There were sighting of unknown submarines in Swedish waters and reports of underwater radio transmissions in Russian. The Russians denied any involvement, however, and there was little clear evidence.

When news of this submarine was announced this week, speculation about Russian activity in Swedish waters quickly reappeared: In initial interviews Monday, Dennis Asberg from Ocean X Team said that it appeared that the submarine was modern, perhaps from the 1980s or 1990s. Tomas Ries, a lecturer in security policy at the Swedish Defense University, told the Expressen newspaper that it appeared to have been on a secret mission that had gone wrong. The Swedish military announced that it was investigating but refused to comment.

However, a closer inspection has led some experts to believe that the ship is not modern. In fact, it may predate even the Cold War by half a century: Russian news site Lenta.ru noted that the cyrillic lettering seen in videos of the wreck include letters used in a way that was phased out with the Russian orthographic reform of 1917-1918. After reviewing the footage, Per Andersson, a retired colonel in Sweden's coastal defense force, told Expressen that he was "very confident" that the submarine was a Russian Som-class submarine (better known by its nicknamed Catfish) that sank in 1916.

On Tuesday, the Swedish armed forces released a statement that said they supported this theory, saying it the wreckage was "most likely" that of the Russian Catfish vessel that had sunk after a collision with a Swedish vessel during the World War I. Ocean X Team's Twitter account confirmed that this could be a possibility, but said more investigation was needed.

It certainly wouldn't be the first time that supposed evidence of a Russian submarine in Swedish waters has been a false alarm: In 1995, Sweden revealed that the undetected underwater sounds that had lead to a hunt for Russian submarines were in fact created by minks or other small, otter-like animals. In Russian state media, Sweden's reaction to the submarine provoked mocking. "Here we go again?" read the headline to a dismissive story on Sputnik News that predicted that the submarine would later turn out to be a fishing boat.

However, both countries may be glad if the submarine isn't modern. Sweden may well be keen to avoid open conflict with Russia – but many Russians still bitterly remember how 118 people died aboard a sunken Russian submarine in 2000.

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