And yet many are asking: Could it have been responsible for the explosion that sank a South Korean warship in March? And if so, how could a sub have slipped through the defenses of South Korea, which, with significant American backing, maintains a fleet far more sophisticated than its northern neighbor's?
Evidence collected thus far indicates a torpedo hit the Cheonan, killing 46 sailors, and suspicion is growing that it was launched from a small North Korean submarine. That scenario would make it the most serious attack on the South Korean military since the peninsula's war ended in a truce in 1953.
"While the North Korean submarine force reflects dated technology by Western standards, North Korean submarines during wartime would present significant challenges, particularly in coastal areas," according to the Arlington, Virginia-based Global Security think tank. "North Korea has placed high priority on submarine construction programs, which are ongoing despite its economic hardships."
Without witnesses or communications traffic to use as evidence, proving North Korea was behind the attack is difficult.
Still, teams conducting an intensive salvage and analysis mission are beginning to put the pieces together.
Officials say they know the 1,200-ton warship - a small, lightly armed frigate that split in half while on patrol in waters near the Koreas' tense western maritime border - sank after a powerful external blast created a shock wave of the sort normally associated with a torpedo or mine.
South Korean media have reported that traces of the high explosive RDX have been found in the wreckage, which would also be consistent with a torpedo attack.
"It is plausible that the ship was hit by a torpedo," Joseph Bermudez, a North Korea military expert and senior analyst for the London-based Jane's Information Group, told The Associated Press.
North Korean subs are not state-of-the-art. Instead, they underscore impoverished North Korea's focus on "asymmetric" warfare - the use of stealthy, relatively low-cost weapons that many a ragtag fighting force have proved can open up big holes in conventional defenses.
The "vast majority" operated by the North Korean navy and intelligence agencies are capable of carrying torpedoes and sea mines, as are some of the intelligence agencies' semi-submersible infiltration landing craft, Bermudez said.
"If the sinking was caused by a torpedo, then I would say that this was a deliberate act of aggression," Bermudez added.
Investigation results are expected within weeks, reports say, and Seoul has been extremely cautious in its comments on the sinking. It initially said there was no indication the North was to blame, and publicly fingering the North appears to hold little upside for Seoul, at any rate. Pyongyang has denied any role in the disaster.
But the idea that a North Korean submersible may have slid so close to the Cheonan undetected has been a wake-up call for the South, which has vowed to strengthen its defenses against low-tech, asymmetric warfare. On Sunday, Seoul set up a task force to review and revamp its defenses.
Many South Korean experts had previously thought that such subs were unable to launch effective attacks, and were of more use for simply crossing the border.
"It shows that both the South Korean and U.S. surveillance and reconnaissance missions either failed or were not in operation in the area where the incident took place," Tong Kim, a visiting professor at Korea University in Seoul, said. "Apparently there was no signal or geospatial intelligence on the movement of a North Korea submarine, if it was involved in the incident. The Cheonan's submarine detector must have failed."
It would not be the first time North Korean submarines have been used to harass or spy on the South.
In 1996, a North Korean submarine ran aground on underwater rocks northeast of Seoul. The 26 commandos aboard tried to flee overland back to the North, but after several skirmishes all but one were killed, along with 17 South Koreans.
Two years later, another submarine was entangled in South Korean fishing nets.
By analyzing the size of the hole bored in the ship, retrieving any residue from the explosion - such as RDX - and looking at tides and currents at the time of the blast, the multinational team of investigators should be able to determine how large the device that caused the damage was and even where it came from.
Already, South Korea's Chosun Ilbo newspaper has reported that South Korean and U.S. intelligence agencies detected a submarine "disappearing and reappearing" at a North Korean base near the site of the downing around the time the Cheonan sank.
South Korean officials refused to comment, as did Japan-based spokesmen for the U.S. 7th Fleet, which joined in the salvage effort and investigation.
But leaders in Seoul have expressed outrage that the country's defenses appear to have been breached.
"We will remember this day as a day of shame," Defense Minister Kim Tae-young said last week, calling the sinking a "surprise attack" that caught the South's military off guard.