South Korean troops could not notice a North Korean man crossing the heavily fortified border this month, although he appeared on TV cameras eight times and caused alarms.
South Korea’s military has said it will take the time to make the necessary changes.
The man swam in a wet suit from the sea, walked 5km and spent three hours unnoticed before troops finally acted on his ninth TV appearance.
It is unclear why he crossed such a dangerous route.
At one point, the man went through a drainage tunnel in the demilitarized zone (DMZ) that the South Korean army did not even know, reports the news agency Yonhap.
This is despite the fact that the zone is one of the strongest amplifiers and sensor dominators in the world.
What happened on the trip?
The latest details come from a report by the Joint Chiefs of Staff of South Korea (JCS) on the decay of security on 16 February. The JCS refused to identify the man and investigate whether he was trying to defect.
It is not known where his journey began, but he landed at 1:05 a.m. with a diving suit and a pinball machine, near a South Korean observatory, north of the eastern border town of Goseong.
He hides the suit and the flippers under a rock and then follows barbed wire fences south along the beach.
He then entered the drainage tunnel and crossed the DMZ.
The man then moves to more than 5 km (three miles) along an unconscious road until 4:16 p.m.
His ninth appearance on a CCTV camera at this point led to troops reporting his presence.
The army found him at 07:27. According to the report, he was looking for civilians to surrender because he was afraid that soldiers would return him to the North.
What was the security breach?
Surveillance cameras spotted the man several times between 01:05 and 01:38, with alarms going off twice. No action was taken.
Military CCTVs caught the man again three times around 4 p.m.
A JCS official told Yonhap that the soldier in charge of the equipment on the coast was making adjustments at the time and that the alarms were a system error.
Some have asked if the man, a citizen working in the fishing industry, swam from North Korea as alleged due to the cold winter conditions. It was speculated that he might rather make the trip by boat.
However, the JCS said it believed the man had swum over and said his wetsuit was in good condition and that the tide would have helped.
This is the second time in four months that questions have been brought to the DMZ about safety.
After a former gymnast crossed over last November by jumping over the border fence, the South Korean military has announced that it will investigate every sensor.
The latest event led to an investigation into the 22nd Division, which is in charge of the DMZ area.
Are deviations frequent?
Since the regime in 2011, North Korean leader Kim Jong-un has reportedly ordered that border controls between the two parties and China be tightened, including by laying more landmines.
But about 1,000 people disappear from North Korea each year and flee from an oppressive state that is confronted by numerous accusations of human rights violations.
Most North Koreans escape by crossing the border into China, where they are in danger of being sent back to the North.
Crossing via the DMZ is dangerous. If the North Korean military sees this and arrests it, those who try to cross over will definitely be taken to a detention center to be questioned. They could be tried and sentenced to long-term in labor camps.
The border and the fortifications have existed at a ceasefire since the Korean War in 1953. North and South Korea remain technically at war because the fighting did not end with a peace treaty.
