The troops of North Korea’s crack invasion units are shorter than the average Western tourist.

I know because I have twice hit my head on the roof of one of the infiltration tunnels that have been discovered beneath the stretch of deceptively peaceful land that divides the Korean Peninsula into North and South.

A camouflage-painted observation point offers tourists a view of the world’s most heavily fortified border.

A South Korean marine keeps a constant lookout into the tunnel as it retreats into blackness to the North, observing through a small embrasure in a thick concrete plug blocking the tunnel. On the surface, 73 meters above our heads, is the most heavily fortified piece of real estate on the planet. And they call it the Demilitarized Zone.

Tensions up here on the border of barbed wire and minefields between the two Koreas are still high — but optimism is breaking out all over the South.

For the first time in more than 50 years, there is a thaw in the air on the peninsula. The governments in Pyongyang and Seoul are on speaking terms and the North has decided to open up its doors and embrace concepts other than juche, its own hardline version of self-dependent socialism.

Or has it? Whisper it quietly: Not everyone you talk to in Seoul is thrilled or convinced by Pyongyang’s charm offensive. Kim Jong Il’s about-face has been just too rapid, too complete, too disarming.

And that, they say, is why the troops on the DMZ have to remain vigilant.

One of the last potential Cold War flashpoint borders in the world, the 249-km border on the 38th parallel has to be one of its most unusual tourist attractions. As my tour bus pulls up in the car park of the Dora Observatory, an elderly Korean woman is sobbing uncontrollably as she is helped aboard her bus. For many not lucky in the lottery for the family exchanges, this is as close as they can get to members of their families that they have not seen for half a century.

From the camouflage-painted observatory (where I am warned three times not to take photos) the view into the North is spectacular. The hillside the observatory stands upon drops away into a lush, uncultivated valley before rising gradually again the far side, the North Korean side. The roads over there are deserted. Nothing moves in the well-maintained villages, and the fields are empty.

After a South Korean marine has given us the description of what we can see beyond the glass (peppered with “unprovoked,” “belligerent” and “purely defensive”) I train the outsize binoculars out into no man’s land and watch a North Korean soldier lean against the parapet of his sandbagged position.

Bizarrely, the tripwires and booby-traps in the 4 km between the two sides have served to make the DMZ one of the safest refuges for indigenous Korean species of flora and fauna. As I watch through the binoculars, a pair of snowy white storks bob and turn in a clearing.

Back on the bus, we pass woodland fenced off and posted with red signs bearing the unmistakable skull and crossbones symbol and the single word “mines” in block capitals. A unit of South Korean soldiers wave us through another check point.

The third infiltration tunnel was discovered in October 1978, crossing the border only 44 km north of Seoul. Military experts claim a full division of the North’s shock troops would have been able to penetrate the South every hour through the tunnel, and that it was designed for a surprise attack on the South’s capital.

To get to the tunnel itself, we descend the steep bore shaft that was drilled to intercept it. Holes to place the dynamite charges are still visible where the North Korean engineers suddenly stopped advancing.

After the discovery of the third tunnel, it was 12 years before another burrow into the South was discovered, this time on the east of the peninsula north of Yanggu.

Yet there are almost certainly invasion routes that have never been discovered and, to this day, may end just a few meters below the surface behind the South Korean front line, ready for the order from Pyongyang for its forces to break through the topsoil and go on the offensive.

To the east of the tunnel lies Panmunjom, where tourists and negotiators alike come face to face with the “duplicitous” and “deceitful” representatives of the North.

We are warned not to speak to, approach or gesture toward the North Korean soldiers in and around the hut where the endless discussions on the future of the Korean Peninsula have been held over the past half-century.

A line runs across the negotiating table, precisely marking the border an ideological separation of parents from children, brothers from sisters, entire families from relatives they have never met.

Corp. Park, part of the South Korean contingent patrolling the border zone, considers himself lucky: all his family are in the South.

“It could happen, there could be reunification,” he says, nodding at the barbed wire defenses. “But it’s a long way off yet.”

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