SEOUL, South Korea -- Kim Kang Yoo was shaking as he passed by his fellow soldiers at North Korea's front-line guard post. A security guard looked at him in the eye, and Kim held on tight to his knapsack. He knew by instinct that if he acted awkwardly, the soldiers guarding the North-South Korea border might notice that he was there not to carry out his assigned mission.
He was there to escape.
He tried to act as cool as possible, and when the guard finally turned away, he started running, frantically, until he was lost from sight in the valley.
It was Sept. 27, 2016, when Kim, now a sophomore political science major at Seoul-based Sogang University, defected through the heavily fortified border that spans the Korean peninsula. He chose to escape the communist regime's military service and flee to South Korea, where he could live a life with the freedom to choose. That journey was the toughest moment in his life.
"To hide myself from the North Korean guard posts' watch, I ran deep into the valley," Kim, 25, recalled. "For hours I wandered in the woods, round and round. I cut myself on a tree branch and almost died falling off a cliff. For the first time in life, I prayed for god's mercy although I had never been to church."
Six North Korean soldiers in the past three years have run for a new life to South Korea by crossing the demilitarized zone, aka DMZ. Kim is one of those courageous young men in their 20s who crawled, bolted, hid his way across the 2.5-mile-wide DMZ, the most heavily armed border in the world, filled with mines from the Korean War era. For some, the life-risking escape took less than an hour, while for others it took days.
In the last three years, 3,682 North Koreans have escaped to South Korea, according to the Unification Ministry. Only six of them -- less than 0.002% -- ran across the untouched area of tension to defect. South Korea's Defense Ministry neither confirmed nor denied the number of defectors through the DMZ. A majority of defectors tend to use routes through a third country like China and Thailand, which is also risky but without the threat of gunfire from colleagues at the DMZ.
The DMZ is a 160-mile-long stretch of land dividing the Korean peninsula since the Korean war paused in 1953. With the symbolic 38th parallel in the middle, soldiers from each side monitor the opposition, keeping in mind that the Korean war never officially stopped. The two Koreas agreed to a ceasefire that's so far lasted for over 60 years.
Soldiers working at guard posts along the border are not only given the role of surveilling the opposition, but also keeping an eye on their own colleagues, who may attempt to escape. All soldiers on each side carry loaded guns.
"I witnessed a soldier, not so different from myself, being executed by firing squad when he was caught trying to flee," An Chan-il, a 66-year-old defector and North Korea expert, told ABC News. He said that he kept the dead soldier's badge during his own escape to at least give the man's soul a chance to have a look at South Korea.
An escaped the communist regime on July 27, 1979. The rainy season had just begun so the weather was foggy and nippy. He believed that heavy fog could help him hide.
In order to prevent intrusion into the DMZ, layers of barbed-wire fence stand on the North Korean side, warning soldiers not to come too close. At major points, the wire fence exposes electricity strong enough to bounce off an adult. An, a first sergeant, was in charge of the button that turned the electricity on and off.
"When the commander called that evening to turn off the electricity for patrol purpose, I took the chance," An said. "I told my colleague not to turn it back on until I came back and exited the guard post. I ran for South."
As a soldier in charge of wire fence, An remembered where the hidden emergency door was. He smashed the lock with his rifle while there was still no electricity. He ran toward the South Korean guard post to his right and stayed low until the sun came up the next morning.
Unlike An, who knew how to avoid electricity running through the barbed-wire fence, Kim chose the ragged valley to avoid mines buried in fields and looked for cracks under the fence that were safe enough to get through. He assumed that electricity won't be running on the fences above water.
"I dug into the soil to make room just enough for myself to pass through," Kim said, remembering the day of defection as the hardest moment in his life. "After running and crawling in the mountainside for more than four hours, I was so exhausted that I even thought of killing myself with [the] rat poison I had in pocket for emergency."
After Kim crossed two fences, he finally arrived at the Bukhan riverside. He crossed the river on foot where the water level was only up to his waist. He remembered the width of the river to be about 70 meters [about 230 feet] where he crossed it.
"I was relieved for the first time that I made it to the other side when I crossed that river, because there were words back in the North Korean army that it was South Korea just across that river," Kim said.
To Kim's dismay, he confronted yet another fence on the South Korean side. This time it was so thick and the roots were so strongly buried to the ground that there was no way he could dig a hole underneath. Instead of finding a way around it, he decided to climb over the fence. In doing so, the wires scratched his arms and ripped his uniform. Exhausted, Kim laid himself in front of the South Korean guard post speechless until the soldiers noticed him.
While it took over a day for An and Kim to arrive in South Korea, it took only a few hours for Han Yengsoo to defect. Han, now a 45-year-old subway operator, defected June 12, 1995. It only took him three hours to cross the DMZ and knock on the South Korean guard post on what he described as a sunny day.
"A number of coincidences helped my defection that day," Han explained. "For the first time since I started military service, I was sent to the DMZ by myself to collect edible greens, which let me escape free from colleagues. And there was no electricity on the wire fence at the time due to a power shortage in North Korea. Plus, I did not step on mines, which could have killed me on the spot."
As Han stood at the 38th parallel, he left one foot on the North's side and the other on the South's for half an hour, trying to decide which would be better for his future -- returning home or starting a new life in South Korea. He chose South.
"I threw pebbles to knock on the guard post gate to let them know that I defected from North," Han said. He found out later that the local media called his case a "knock defection."
Han told ABC News that at first he thought he could easily return to North Korea if he didn't enjoy life in South Korea. He was already well off in North Korea, with a predictable life ahead of him. But he realized after a seven-month-long investigation at the national intelligence that defection was a one-way trip.
The North Korean army is fully aware that soldiers who stare at the southern side all day long are exposed to propaganda flyers and broadcasts from the south. That's why only young men from relatively wealthy families or well-recognized backgrounds are assigned to work near the border, Han said.
"Those who are assigned service at the border are mostly from good backgrounds," Han said. He noted that he, too, came from an established family, with his father being dispatched to Libya as a dentist at one point and his mother a school teacher in North Korea. "The army assigns soldiers who can have affluent lifestyles in North Korea, so that there is less temptation for flight."
Similarly, Kim's hometown was Pyongyang and his father worked as a secretary in a steel factory. An was already a Worker's Party member when he defected -- his father worked as an official for the Kim Il Sung regime.
North and South Korea pledged to demilitarize the joint security area in Panmunjom after the Pyongyang inter-Korean summit in September 2018. Since October, security guards from both sides work unarmed at the Panmunjom. North and South Korea's defense each got rid of 10 out of 11 guard posts built along the border, confirmed by each sides in December.
ABC News' Joohee Cho, Hansol Park and Sorah Choi contributed to this report.