Since the end of World War II, the United States has routinely employed ships and aircraft on spying and observation missions of varying legality—and every now and again, something has gone wrong. A too-stealthy American submarine bumps into a Russian counterpart, a spy ship off Korea gets seized, a U-2 spy plane gets shot down, or a Navy P-3 collides with a Chinese fighter and is forced to land in Chinese territory. In the event the spies can’t return to home base, they’ve mostly surrendered to local troops and were eventually repatriated after interrogation and diplomatic wrangling.

In September 1996, it was the turn of a North Korean spy submarine to experience such a mishap. But due to the North Korea’s fanatical military culture, what could have ended as a diplomatic embarrassment ended in a tragic bloodbath.

At 5 a.m. on September 14, 1996, a North Korean spy submarine commanded by Capt. Chong Yong-ku slipped out of its base in Toejo Dong. The thirty-four-meter-long Sang-O (“Shark”) normally had a crew of only fifteen. This time, however, it carried a special cargo, including a team of three special forces operatives from the elite Reconnaissance Bureau, accompanied by Col. Kim Dong-won, director of the unit’s maritime intelligence department.

At the time, North Korea was in the midst of a devastating famine that would claim hundreds of thousands of lives. This only inspired Pyongyang to grow more paranoid that South Korea, with which it had never declared peace, would exploit its disastrous condition. Before departing, the crew of the submarine had sworn an oath not to return home without completing their mission: to spy on the South Korean military bases around the area of Gangneung, ninety miles south of the Demilitarized Zone (DMZ) separating the two countries.

Captain Chong’s mission was relatively mundane as North Korean special operations went. Another submarine had performed the same mission exactly a year earlier. During the 1960s and 1970s, North Korea had infiltrated thousands of operatives into South Korea, many of whom died on sabotage and assassination missions targeting South Korean leaders. North Korea also pursued a program of abducting civilians off the coast of Japan to serve as language instructors.

The little submarine arrived a few hundred meters off of Gangneung the following day. Around 9 p.m., the special operatives swam ashore in scuba gear, accompanied by two divers to provide assistance. The infiltrators proceeded inland to pursue their mission, while the divers returned to the submarine, which crept back along the coastline to photograph South Korean military installations.

The following evening, the mini submarine returned to recover the special-ops soldiers. But something had gone wrong, and the infiltration team was nowhere to be found. The submarine withdrew to the sea, and again attempted to recover the spies the night on the seventeenth.

This time, though, the submarine ran aground on a rocky reef around 9 p.m. The 325-ton boat came to a rest just twenty meters off of An-in Beach, three miles away from Gangneung, its screw jammed with seaweed. The crew feverishly attempted to dislodge the vessel to no avail. Finally, Captain Chong gave the order to abandon ship near midnight, setting fire to the interior of his vessel before disembarking with his crew.

As fortune would have it, at 1:30 a.m. that morning a passing South Korean taxi driver noticed the silhouette of the stranded submarine in the water—and the nearly two dozen men assembled near the beach. He alerted the South Korean military, which dispatched police and soldiers to investigate. By 5 a.m. the South Korean military had all of Kangwon Province on alert. The abandoned submarine was boarded at 7 o’clock that morning, and soon more than forty-two thousand troops from the Eighth Corps and the Thirty-Sixth Infantry Division were mobilized to hunt down the missing crew, assisted by helicopters and police tracking dogs. The Republic of Korea Navy organized a blockade in case additional submarines were present.

That afternoon, a farmer reported a strange man walking in his fields. South Korean soldiers descended upon the area and managed to capture the submarine’s thirty-one-year-old helmsman, Lee Kwang-soo, at 4:30 p.m. Lee claimed his submarine had experienced an engine failure while on a training mission, causing it to drift into South Korean territory. He did not mention the presence of the Special Forces operatives.

Just a half hour later, South Korean troops made a horrifying discovery on the top of a nearby mountain—the bodies of ten men in a neat row, dressed in white civilian tee shirts and tennis shoes. Among them was Captain Chong and members of the submarine crew. An eleventh victim, Colonel Kim, lay dead on his side a short distance away. Every one of them had been shot in the head at short range. The government subsequently instituted a curfew across the entire coast.

Meanwhile, the interrogation of Lee Kwang-soo progressed, assisted, as legend has it, by four bottles of soju, the popular mild Korean liquor. Lee confessed that his boat had been involved in an espionage mission, and noted the crew had been instructed “to commit suicide to avoid capture.” The dead crewmen had been executed because they were “not strong and might have been captured.” It’s thought their deaths may have a punishment for their accidental grounding of the sub, or due to their lacking the combat skills necessary to escape back to North Korea.

Soo also revealed an important fact: his submarine had carried a total of twenty-six men, including the Special Forces personnel. This meant fourteen infiltrators were still unaccounted for. Starting at 10 a.m. the following morning, South Korean troops searching around the mountain lands around Gangdong-myeon engaged in the first of three firefights with dispersed teams of North Korean crew, killing seven by that afternoon at the cost of two wounded. Another four were killed in gun battles by the end of September, their bullet-riddled bodies displayed to the South Korean media, while one of the infiltrators killed a South Korean police officer on the twenty-ninth while he was leaving work in Gangbori. The three elite Reconnaissance Bureau operatives, however, were still on the run.

South Korean president Kim Young-sam had issued a statement on September 20 that he might be forced to retaliate if there were further provocation. Pyongyang replied that its spy sub had “encountered engine trouble and drifted south, leaving its crew with no other choice but to get to the enemy's land, which might cause armed conflict.” It also threatened retaliation for the deaths of the crew. When South Korean consular officer Choe Deok-geun was assassinated in Vladivostok on October 1, it was generally believed his death was arranged in revenge for the crew. The poison used to kill Choi was identical to the type found aboard the captured North Korean submarine, which by then had been towed to Tonghae for inspection.

The hunt for the North Korean agents would last forty-nine days as they sought to escape across the DMZ. On October 9, police found the bodies of three civilians who had been picking mushrooms near Tongdang-ri. Spent 5.56 millimeter casings from M16 assault rifles were found close to their bodies. Two weeks later, an off-duty Korean soldier was strangled to death .

Finally, on November 4, a civilian driver spotted two strange men crossing a highway near Inje, just twelves miles short of the border, and called the police. The following morning, South Korean troops cornered the two agents in a running gun battle on Hyangro Peak. The North Korean operatives responded with blazing M16s and more than a dozen hand grenades, killing three ROK soldiers before being shot to death. A diary found on their bodies recorded their killings of civilians and their journey across nearly eighty miles of South Korean territory.

This marked the end of the manhunt, which cost the province over 200 billion won ($187 million) dollars in economic damage. The North Korean spies killed four civilians, eight soldiers, a policeman and a reservist attempting to escape. In return, of the twenty-six men aboard the submarine, only two remained alive. The third North Korean special force soldier, Li Chul-jin, is believed to have escaped.

On December 29, the North Korean government offered a rare statement of regret for the incident. In reciprocation, Seoul repatriated the cremated remains of the twenty-four North Korean agents the following day—the first ever such exchange between the two Koreas. Unfortunately, Pyongyang’s habits had not truly changed. Another one of its spy submarines would meet a terrible—and again, avoidable—fate a year and a half later off the coast of the South Korean city of Sokcho, but that is a tale for another time.

The failure to detect the spy submarine led to a shakeup of the Republic of Korea military, with twenty officers disciplined and two general relieved of their posts. In 2011, the South Korean military even staged a military exercise recreating the circumstances of the incident, in order to test whether it could respond more effectively.