THESE pictures reveal the inside of invasion tunnels dug by North Korea - which Kim Jong-un could use to send as many as 30,000 troops an hour to the South in a stealth attack.

Fears have been raised that as many as 84 secret war tunnels have been built, with some boasting sleeping areas, a railway and enough space to move tanks.

9 A soldier is pictured outside the entrance to one of the tunnels - which it is feared Kim Jong-un could use to send 30,000 troops an hour beneath the North Korean border Credit: Getty Images

The existence of these sprawling tunnels has raised the risk of a surprise invasion amid heightened tensions between the North and South.

The first major tunnel, which stretches 3.5km in length, was discovered in November 1974.

It was found north-east of Korangpo in the Korean Demilitarized Zone (DMZ), extending 1km past the official border.

9 Fears have been raised that as many as 84 secret war tunnels have been built Credit: DMZ Tours

9 The entrance of an invasion tunnel under the DMZ between South and North Korea Credit: Getty Images

The walls, which are 1.2m high and 1m wide, are reinforced with concrete slabs.

The tunnel has the capacity to move one regiment an hour – which means between 10,000 and 30,000 elite soldiers could pour through the cramped space.

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It was equipped with a narrow-gauge railway and rail cars and was lit with lamps connected to 220-volt power lines.

A second tunnel was found in March 1975, 13km north of Chorwon, which stretched 1.1km south of the heavily-fortified border.

9 This shows the entrance to one tunnel that was found in October 1978 Credit: Josh Berglund/Wikipedia

9 A soldier is seen standing guard outside the entrance of this tunnel Credit: Getty Images

Twice as wide as the first tunnel, it is large enough to send through heavy weapons, tanks and field artillery.

Troops would be able to march through the underground tunnel at three to four abreast – meaning thousands could be deployed in an hour.

The second tunnel ran for 3.5km at a depth of 160m and is it understood to be 2.2m wide.

A third tunnel was uncovered after a tip-off from North Korea defector Kim Bu-seong on 17 October 1978.

9 It is feared that North Korean despot Kim Jong-un could use the tunnels to launch a surprise attack Credit: AP:Associated Press

9 This tunnel, stretching under the border with South Korea, was fitted with electricity Credit: Getty Images

It runs for 1.6km, some 73m below the surface, just south of Panmunjeom.

After it was built, coal dust was smeared around the entrance so it would be disguised as an abandoned coal mine.

It is now blocked by three concrete walls – but it still has enough room to transport 30,000 troops per hour.

Another tunnel was discovered in March 1990 which runs for 1,052m.

The Daily Star reported that the South Korean government believes there could be 16 more undiscovered tunnels.

A retired South Korean general has previously claimed there were as many as 84 tunnels beneath the border.

But the South dismissed the claims made by Hahn Sung-chu, a former two-star general, saying they were "groundless".

9 A second tunnel was found in March 1975, 13km north of Chorwon, which stretched 1.1km south of the heavily-fortified border Credit: Bettmann