A maritime mystery is rattling the usually quaint Swedish capital as authorities are trying to determine what, exactly, is lurking in the waters off Stockholm.

The Swedish military had overheard a distress call, presumably from a Russian submarine, on Friday, local media reported. By the end of the weekend, the navy had made "three credible sightings of foreign undersea activity" near the Stockholm coast, according to the AP.

The reports fueled speculation about a scenario reminiscent of the 1990 Tom Clancy action thriller The Hunt for Red October, prompting clarifications from the Swedish prime minister. "There is no submarine hunt under way," AFP quoted Stefan Loefven. "There is an ongoing intelligence-gathering operation."

Nevertheless, the military on Monday intensified its search in the vicinity of Stockholm, and the government issued airspace restrictions and warned the public to keep a distance. ""There's an increase in military exercises from both the Russian and the NATO side," Loefven concluded.

Moscow, meanwhile, flatly denied that any of its subs had been operating in Swedish waters. The state-run news agency TASS quoted a Defense Ministry official as mocking Stockholm's search for a "non-existent Russian sub." The Russian official suggested a Dutch vessel may be to blame for the incident, a claim the Netherlands promptly denied, Voice of Russia noted.

The latest report, meanwhile, suggested a less sovereign culprit on Wednesday: Lasse Schmidt, a local tour guide who "putters around the archipelago with tourists in a small decommissioned military sub originally designed as a target for naval battle exercises," according to CNN.

Whatever the backstory - and outcome - of the saga, it already holds a lesson for Stockholm: The hunt, the AP titled, points to a "readiness gap" for a once "robust anti-submarine force." So Sweden may have to heed the advice of Red October's Capt. Ramius: "You're afraid of our fleet. Well, you should be."