A North Korean soldier defected to the South by walking across the heavily protected Demilitarised Zone (DMZ) separating the two sides.
This is the third defection by a North Korean soldier via the DMZ in the last three years.
But how do you get over one the world's most heavily guarded strips of land without being spotted?
The North Korean soldier, who has not been officially identified, approached a South Korean guard post to surrender himself at around 19:50 (10:50 GMT) on Tuesday.
"We are holding him to investigate the motive and the process of his defection," the defence ministry said in a statement.
There was no exchange of fire during the incident, according to the South's Office of the Joint Chiefs of Staff (JCS).
It said the soldier crossed the military demarcation line (MDL) in the middle sector of the DMZ.
The last time a North Korean soldier defected via the DMZ was in September 2016 and before that, in June 2015.
In 2012 two soldiers from the North made it through the dense security net and handed themselves over.Image copyright Getty Images Image caption The fortified border is almost impenetrable
Is it dangerous to cross the DMZ?
Yes. The DMZ is a strip of land 250km (155 miles) long and 4km (2.5 miles) wide that runs across the Korean Peninsula, heavily mined and fortified with barbed wire, rows of surveillance cameras and electric fencing.
It is also closely guarded by tens of thousands of troops on both sides, making it almost impossible to walk across.Image copyright AFP Image caption Three defectors successfully crossed the military demarcation line in the last three years
Swathes of bare land are littered with large rocks and anti-personnel landmines.
If the North Korean military spot movements across the area, it is likely that they will open fire.
The border and its fortifications have been in place since the Korean War ended in an armistice in 1953. North and South Korea remain technically at war as the fighting did not end with a peace treaty.
Since he took power in 2011, North Korean leader Kim Jong-un is believed to have ordered tightening border control between the two sides and with China, including by laying more landmines.
In recent months, North Korea has also flown drones over the border, mainly for reconnaissance purposes in the wake of South Korea's deployment of the US anti-missile defence system, known as the Terminal High Altitude Area Defense (THAAD).
How often do North Koreans defect?
On average, around 1,000 people from the North flee to the South each year but only a handful picked this highly dangerous escape route across the military demarcation line (MDL) during the last decade.
Successful defection cases prove that it can be done. However nobody knows the number of unsuccessful attempts made by desperate defectors-to-be.
If spotted and arrested by the North Korean military, those trying to cross the DMZ would certainly be taken to a detention centre to be interrogated. They could be tried and sentenced to lengthy terms in labour camps.
One of the two North Korean defectors in 2012 had to kill his platoon commanders before fleeing.
But the crossing is sanctioned by both sides, not only the North.
In July 2012, South Korean officials arrested an activist, Ro Su-hui, after he walked back across the border from visiting the North to promote reunification.
The first South Korean unification activist to cross the border was Lim Su-kyung, who visited North Korea in 1989 and was jailed after returning home.Image copyright Getty Images Image caption Both sides blast music and propaganda across their shared border
What happens in the case of successful defection?
Defectors usually approach South Korean border guard posts to express their intention to defect. But there are also telephones on the South Korean side so those who have fled from the North can call seeking help.
Most North Korean defectors choose to flee using easier routes than over the DMZ.
Seoul says more than 30,000 North Koreans have defected to the South since the end of the Korean War, the majority via China, which has the longest border with the North.
Some defectors travel on to countries in Southeast Asia and elsewhere in the hope of resettling in South Korea or a third country.
After reaching the South, most North Korean defectors are first held at an interrogation facility to screen for potential spies and then put through a state resettlement programme.
They can get help from the government and there are also non-profit and non-governmental organisations that seek to make the transition easier for them. However this is a process that often proves not only difficult but also traumatic.
Some escapees find that escaping North Korea is just the start of their journey. They then have to try to cope with the brutality of the regime, and the years of physical and psychological hardship they faced.