DMZ infiltration units...?
Backwards and broke, Pyongyang long-ago abandoned any idea of winning a conventional war with the South and their American allies... that is, ever since they saw US forces cut Saddam's armor into tinsel with shock-n-awe back in 2003.
So to remain a credible threat, the Norks rely on a three-pronged strategy of heavy artillery at the DMZ to pound Seoul into smithereens along with missiles that can menace the rest of the ROK... this while building the world's largest special forces, fiercely-loyal and highly-trained in asymmetrical warfare and infiltration strategies including Islamic-terrorist-inspired car bombs, roadside explosives, etc.
The ambitious plans of the DPRK's Special Operations Force include myriad psy-ops, infiltration tunnels under the DMZ, up to 7000 commandos landing aboard midget subs and hovercraft on both southern coasts, others bypassing the DMZ in radar-evading low-tech wood-packed biplanes, troops simply walking over the border in South Korean uniforms, even Nork agents flying-in dressed as businessmen aboard Chinese commercial flights... all to wreak havoc and terror behind the lines of US and SK troops as they create a second front in the event of full-scale conflict [think "suitcase nuke" and you'll get an idea of the potential of the threat posed].
The DPRK has stationed 50,000 of these troublesome forces right at the DMZ, a number equal to all US special forces combined. Total North Korean special forces are now estimated at 200,000+...over double what they had just three years ago.
Unsettlingly it is then that today SK border surveillance is finding North Korean special forces troops in new camouflage gear apparently preparing for DMZ intrusion drills. Coming on top of recent escalations in the 60-year-old Korean War that has never officially ended, this offers reason for concern... although Seoul won't be standing idly by while Dear Leader plans their demise.
The vile Stalinist regime in North Korea has always held an affection for special forces/asymmetrical warfare strategies, given that "Great Leader" Kim Il Sung fought as a guerrilla vs the Imperial Japanese in occupied WWII Korea and DPRK propaganda is steeped in the narrative.
The most famous incident involving such elite Korean People's Army troops would be the 1968 infiltration that had 31 North Korean commandos of the KPA's secret special forces Unit 124 on an assassination mission- one only halted mere meters from the South Korean President's "Blue House" residence near Seoul. Most of the Norks were killed in the ensuing gun battle with SK security personnel, but the group's leader was actually captured alive.
And one of them did escape alive and return to North Korea, highly decorated and rewarded in spite of the mission's failure: Park Jae-gyong is today an aide to Kim Jong-il and vice minister of the North Korean People's Armed Forces...
More on recent developments in the escalating
North Korea - South Korea conflict -here-