This satellite image released this month shows the Punggye-ri nuclear test site in North Korea. Picture: Airbus Defense and Space/38 North/Pleiades CNES/AP

IT boasts a complex series of tunnels and testing infrastructure in the remote mountains of northeast North Korea.

And while satellite images provide some clue as to what the North Korean regime is up to, few outside the secretive state would really know what goes on underground here.

As North Korea continues to pursue its ambition of putting a nuclear warhead on a missile system capable of reaching targets around the globe, all eyes are on Punggye-ri.

This is where North Korea tests its nuclear weapons and remains one the country’s most strategically important sites.

Individuals are seen walking near the guard barracks and security checkpoint at North Korea’s Punggye-ri Nuclear Test Site. Picture: Airbus Defense and Space/38 North/Pleiades CNES/APSource:AFP

Just this week North Korea watchers said they had observed unusual activity at the reclusive country’s nuclear test site with workers reportedly playing volleyball.

The 38 North monitoring group said satellite imagery captured on Sunday showed personnel at the guard barracks and two other areas at the Punggye-ri test site playing the popular game.

Joseph Bermudez, a North Korea expert and an analyst for the Washington-based 38 North said this could suggest the facility is going into standby mode.

The group correctly predicted the most recent test in September last year.

North Korea has carried out five nuclear tests — two of them last year — and multiple missile launches.

Experts examining satellite imagery found a wide range of activity, including tunnel excavations at Mount Mantap, a mile-high peak where the regime conducts its nuclear tests.

Mount Mantap is a nondescript granite peak in the remote and heavily forested Hamgyong mountain range about 80 kilometres from Chongjin, the nearest big city.

North Korea is the only country in the world that still conducts nuclear weapons tests and its Punggye-ri site mostly — under Mount Mantap — is also the world’s only active nuclear testing site.

The images released last week showed new activity in the Main Administrative Area and a few personnel around the site’s Command Centre. Picture: Airbus Defense and Space/38 North/Pleiades CNES/APSource:AFP

“It also suggests that these volleyball games are being conducted with the North Koreans knowing that we will be looking and reporting on it,” Mr Bermudez told AFP.

“They are either sending us a message that they have put the facility on standby, or they are trying to deceive us.”

Just last week, 38 North last week said Punggye-ri was “primed and ready” to conduct its sixth nuclear test, possibly to coincide with last Saturday’s celebrations marking the birthdate of regime founder Kim Il-sung.

But that test didn’t take place.

However North Korean leader Kim Jong-un attempted to launch a missile on Sunday that the Pentagon said blew up almost immediately after launch.

North Korea is ready to launch a nuclear test at its Punggye-ri Nuclear Test Site, according to the 38 North monitoring group. Picture: Airbus Defense and Space/38 North/Pleiades CNES/APSource:AFP

Commercial satellite imagery of the test site from April 12 shows continued activity around the North Portal, new activity in the Main Administrative Area, and a few personnel around the site’s Command Centre,” the 38 North said.

The images also showed several mining carts had deposited waste onto larger piles by the underground testing facility’s north entrance.

READY FOR TESTING

According to John Hallam, a UN nuclear disarmament campaigner, the satellite images show that the site is technically ready for a test.

But Mr Hallam said that didn’t mean to say North Korea was ready to conduct a test tomorrow adding the site has actually been technically ready for quite a few months.

He said the North Koreans would be aware satellites were monitoring the station and it was entirely possible that the volleyball action captured on satellite wasn’t all it appeared to be.

“I wouldn't put deception past them,” he said.

The site has been ready to test for some time. Picture: Airbus Defense and Space/38 North/Pleiades CNES/APSource:AFP

Mr Hallam, who is a member of the People for Nuclear Disarmament and the co-founder of the Human Survival Project, said the reality was North Korea would test when it was good and ready.

“I have noted over the years that there is a pattern in which the DPRK is ‘ready to test’ according to satellite photos and then three months later it is again ‘ready to test’, and then when the attention goes off how it is ‘ready to test’ it tests,” he said.

“North Korea could test tomorrow. Or in six months time.”

WHAT LIES UNDERNEATH

Mr Hallam said it was difficult to determine exactly how far the tunnels stretched or how big the facilities were with large sections being deep underground.

A South Korean marine watches a TV screen reporting about North Korea's possible nuclear test in September last year. Picture: Kim Ju-sung/Yonhap/APSource:AP

But Mr Hallam said the range of tunnels and their depth would make it difficult to bomb or attack the site.

“The tunnels are perfect for tests — it’s hard to bomb something that’s underground,” he said.

Huge craters built underground could absorb a large test but without reading their minds it was difficult to determine what the North Koreans planned to do, he said.

MONITORING SYSTEMS

If and when a nuclear test was conducted, Mr Hallam said sophisticated monitoring systems would pick up even the faintest trace of a test.

Technology could even detect traces of gases and ground movement.

“These are highly sophisticated monitoring systems,” he said.

However the test is very likely to be done underground and it could be difficult to determine just how big it is.

Mr Hallam said a lot of geological and geophysical analysis is used to arrive at a figure for how big a test the site itself will take.

“Much of it is also history of the site and the tunnelling, which is complicated,” he said.

“My guess is that it might be 20-30 kiloton, maybe as much as 40kt.

“And of course it might be a dud.”

debra.killalea@news.com.au