The man, who has not been identified by South Korean authorities, crossed the Demilitarized Zone (DMZ) from North Korea on February 16, according to the South’s military.
The grainy video footage, obtained by South Korean broadcaster TV Chosun, which Reuters cannot independently verify, shows a dark figure passing under a road sign as bright lights flash.
Wearing a diving suit and fins, the man had swum for around six hours in the ocean in the dead of night, coming ashore more than 3 km (1.9 miles) inside South Korean territory, the Joint Chiefs of Staff said this week in a report cataloguing a series of failures by border security troops. The man then passed under a fence by climbing through a drainage tunnel that security forces had been unaware of, the report said.
Despite triggering alarms and appearing on surveillance cameras multiple times, the man was not noticed until around three hours after he came ashore, the JCS said. It took troops several more hours to track him down and detain him.
The delays in detecting the man drew new criticism of border security, after a similar publicly known case in November, when a North Korean man defected to the South via the eastern DMZ.