It may have drifted off course into enemy waters. More likely it was on a clandestine mission, picking up or depositing one of the spies who are believed to regularly infiltrate the South.
Either way, its mission was never accomplished. As the submarine attempted to surface, its periscope and propeller became hopelessly tangled in the nets of a South Korean trawler.
After the fishermen raised the alarm, South Korean helicopters, spotter planes and warships were quickly on the scene.
Last night, after being towed all day through the Sea of Japan, the 70- ton, Yugo-class sub was brought to land at the port of Tonghae, where it slowly sank in 100ft of water. Sonar scans of the vessel's hull detected no signs of movement, although such submarines typically carry a complement of six to 10 people.
"It probably means that the inside is filled with water and that the crew perhaps drowned or suffocated due to lack of oxygen," said Major General Lim Jong Chun of the South Korean Joint Chiefs of Staff. "The crew may have escaped before the navy was called".
Unconfirmed reports said that a large bang was heard coming from the vessel as it was being towed, suggesting that the occupants may even have blown themselves up.
For a country like North Korea, whose official propaganda boasts endlessly of its military might, this would be bad enough in itself, but it is not an isolated event.
In September 1996, in the same area of sea and coastline, a larger submarine ran aground, leaving its crew stranded. The massive manhunt that ensued left 24 North Koreans and 13 southerners dead. Relations between the two countries remained tense for months.
But things have changed since then. In a sign of the new maturity in inter-Korean relations, both sides appeared concerned to play down the latest incident.
North Korean radio, which usually misses no opportunity to execrate the Seoul government, reported the submarine's fate in unusually calm and detailed terms. The submarine was "wrecked while in training" after experiencing problems with its observation and surfacing systems, it said.
In the South, a stern Major General Lim said: "The fact that the submarine infiltrated across our coastal sea areas is a clear armed provocation and a violation of the armistice agreement." But a spokesman for the South Korean president, Kim Dae Jung, whose "sunshine policy" of tolerance and engagement has improved relations since his inauguration in February, was more conciliatory.
"The submarine incident will not shake our sunshine policy," he said. "The government will try even harder to embrace the North ... with patience."
In other ways, yesterday was a bright day in inter-Korean relations. For the first time in seven years, British, American and South Korean army officers representing the United Nations held talks with their North Korean counterparts in the demilitarised zone (DMZ) between the two countries.
Meanwhile, South Korea's most famous businessman, Chung Ju Yung, the founder ofHyundai, returned to the South after an eight-day visit, during which he presented the famine-stricken North with a gift of 500 cattle and reached an agreement to promote tourism from South to North.
Only relatively small numbers of troops appear to have been mobilised to deal with the submarine - in contrast to 1996 when the then South Korean president, Kim Young Sam, dispatched 60,000 soldiers and police to hunt down the escaped crew members and put parts of the country on a virtual war footing.Reuse content