Since 1974, four tunnels have been found running under the DMZ, dug by the North Koreans so that their army could launch a surprise attack. Walking along 265m of this 73m-deep tunnel is not for the claustrophobic or the tall: creeping hunched over, you’ll realise why visitors get issued hard hats. The guide will point out how the North Koreans painted the rocks black so they might claim it was a coal mine.