Satellite images captured in recent weeks suggest that North Korea might soon conduct another underground detonation in its effort to learn how to make nuclear arms — its sixth explosive test in a decade and perhaps its most powerful yet.

North Korea’s nuclear tests have grown steadily more destructive, and the country continues to pursue its longtime goal of putting a nuclear warhead on an intercontinental missile capable of reaching targets around the globe.

Its nuclear aspirations have recently heightened tension between the North and the Trump Administration, prompting Secretary of State Rex Tillerson to announce that the “policy of strategic patience has ended."

On Sunday night, Xi Jinping, the president of China — which is one of North Korea’s closest allies — urged Mr. Trump through a phone call to show “restraint” with North Korea, according to a Chinese television report.

Signs of an approaching nuclear test have been uncovered through satellite images, in which experts have observed a wide range of activity at Mount Mantap, a mile-high peak where North Korea conducts its nuclear tests. Beneath the mountain, a system of tunnels has been excavated for the past five detonations of the North’s nuclear bombs.

Big debris pile suggests the possibility of a much larger detonation.

Since late 2013, piles of rocky debris from the excavation of the site’s north tunnel system have grown quite large — now big enough to cover a football field, and quite high. It’s the largest pile ever observed there. Work on the excavation has recently slowed, quite likely signaling readiness for the next detonation.

Jan. 5, 2017 Jan. 22, 2017 Additional debris Entrance to north tunnel 125 feet Jan. 5, 2017 Jan. 22, 2017 Additional debris Entrance to north tunnel 125 feet Jan. 5, 2017 Jan. 22, 2017 Additional debris Entrance to north tunnel 125 feet Jan. 5, 2017 Entrance to north tunnel 125 feet Jan. 22, 2017 Additional debris Source: DigitalGlobe Imagery

So too, observers of the test site have recently noted a lot of water being pumped out of the north tunnel system – presumably to prevent washouts and keep it dry for test instrumentation. Groundwater is often a problem in tunneling as it can slow progress, weaken structures and cause shorts in electrical gear.

March 28, 2017 Jan. 22, 2017 Entrance to north tunnel Water drained from tunnel Water continued to be pumped out 325 feet March 28, 2017 Jan. 22, 2017 Entrance to north tunnel Water drained from tunnel Water continued to be pumped out 325 feet Jan. 22, 2017 March 28, 2017 North tunnel entrance Water continued to be pumped out Water drained from tunnel 325 feet Jan. 22, 2017 March 28, 2017 Entrance to north tunnel Water continued to be pumped out Water drained from tunnel 325 feet Source: DigitalGlobe Imagery

Scientists at the Los Alamos weapons lab who have studied images of the large debris pile recently concluded that Mount Mantap could withstand a nuclear explosion of up to 282 kilotons – roughly 20 times stronger than the Hiroshima blast. Previously, the largest detonations were in the Hiroshima range.

No one outside of North Korea knows for sure what could take place or how big the blast might be. It’s a guessing game — though a sophisticated one for Washington’s intelligence agencies, and less so for civilians armed only with unclassified information. Either way, it's a detective story full of clues, questions and a protagonist with a clear motive.

Experts in satellite imagery, military analysts around the world, and geologists and physicists track progress at the remote site mainly through the observation of tunneling, building construction, truck movements and, though harder to see, personnel moves.

Seismic activity helps pinpoint North Korea’s detonations.

Mount Mantap is the world’s only active nuclear test site. Most other nuclear states long ago gave up such explosions in a coordinated effort to end arms races and their dangerous and costly spirals of military action and reaction.

In North Korea, the test devices are buried deep inside tunnels bored through solid rock far below Mount Mantap’s peaks, creating field labs for nuclear experiments. The nearest major city, Chongjin, is situated about 50 miles to the northeast.

The tunnels for the North’s tests are excavated just over halfway up Mount Mantap, which rises to a height of 7,234 feet. The bomb is placed at the end of the tunnel, which gets partly backfilled to prevent radioactive leakage, and then detonated in the test.

MT. MANTAP 7,234 feet 7,000 feet 6,500 6,000 Test 5 About 2,625 feet deep Magnitude 5.3 Test 2 About 1,600 feet deep Magnitude 4.5 5,500 North tunnel entrance 5,000 0.25 miles 0.5 1 1.25 MT. MANTAP 7,234 feet 7,000 feet 6,500 6,000 Test 5 About 2,625 feet deep Magnitude 5.3 Test 2 About 1,600 feet deep Magnitude 4.5 5,500 North tunnel entrance 5,000 0.5 1.25 0.25 miles 1 MT. MANTAP 7,234 feet 7,000 feet 6,500 Test 5 About 2,625 ft. deep Magnitude 5.3 6,000 Test 2 About 1,600 ft. deep Magnitude 4.5 North tunnel entrance 5,500 5,000 0.5 1.25 1 0.25 miles MT. MANTAP 7,234 feet Test 2 About 1,600 ft. deep Magnitude 4.5 Test 5 About 2,625 ft. deep Magnitude 5.3 7,000 feet 6,000 5,000 North tunnel 0.25 miles 0.5 1.25 1

To pinpoint the geographic site of the nuclear blasts, experts rely on the kind of seismologic data used to track earthquakes. Similarly, each of the North’s detonations has generated shock waves that register around the globe.

NORTH KOREA Area Shown Pyongyang Test 5 Magnitude 5.3 Sept. 2016 The underground tunnels culminate in a hook, allowing them to seal themselves after the blast. This design is believed to have come from Pakistani nuclear test sites. Test 4 Magnitude 4.8 Jan. 2016 MT. MANTAP Test 2 Magnitude 4.5 May 2009 Test 3 Magnitude 4.9 Feb. 2013 Test 1 Magnitude 4.1 Oct. 2006 NORTH TUNNEL ENTRANCE DEBRIS PILE EAST TUNNEL ENTRANCE 0.5 miles Satellite Image by CNES / Astrium via Google Earth | Magnitude data from the Nuclear Threat Initiative and USGS Note: Tunnel system based upon an analysis by Frank Pabian and Siegfried Hecker. NORTH KOREA Area Shown Pyongyang Test 5 Magnitude 5.3 Sept. 2016 Test 4 Magnitude 4.8 Jan. 2016 MT. MANTAP Test 1 Magnitude 4.1 Oct. 2006 Test 2 Magnitude 4.5 May 2009 Test 3 Magnitude 4.9 Feb. 2013 NORTH TUNNEL ENTRANCE DEBRIS PILE EAST TUNNEL ENTRANCE The underground tunnels culminate in a hook, allowing them to seal themselves after the blast. This design is believed to have come from Pakistani nuclear test sites. 0.5 miles Satellite Image by CNES / Astrium via Google Earth | Magnitude data from the Nuclear Threat Initiative and USGS Note: Tunnel system based upon an analysis by Frank Pabian and Siegfried Hecker. NORTH KOREA Area Shown Test 4 Magnitude 4.8 Jan. 2016 Test 5 Magnitude 5.3 Sept. 2016 Test 2 Magnitude 4.5 May 2009 Test 1 Magnitude 4.1 Oct. 2006 Test 3 Magnitude 4.9 Feb. 2013 MT. MANTAP NORTH TUNNEL ENTRANCE DEBRIS PILE EAST TUNNEL ENTRANCE The underground tunnels culminate in a hook, allowing them to seal themselves after the blast. This design is believed to have come from Pakistani nuclear test sites. 0.5 miles Satellite Image by CNES / Astrium via Google Earth | Magnitude data from the Nuclear Threat Initiative and USGS Note: Tunnel system based upon an analysis by Frank Pabian and Siegfried Hecker. NORTH KOREA Area Shown Pyongyang Test 5 Magnitude 5.3 Sept. 2016 The underground tunnels culminate in a hook, allowing them to seal themselves after the blast. This design is believed to have come from Pakistani nuclear test sites. Test 4 Magnitude 4.8 Jan. 2016 Test 2 Magnitude 4.5 May 2009 MT. MANTAP Test 3 Magnitude 4.9 Feb. 2013 Test 1 Magnitude 4.1 Oct. 2006 NORTH TUNNEL ENTRANCE DEBRIS PILE EAST TUNNEL ENTRANCE 0.5 miles Satellite Image by CNES / Astrium via Google Earth | Magnitude data from the Nuclear Threat Initiative and USGS Note: Tunnel system based upon an analysis by Frank Pabian and Siegfried Hecker. NORTH KOREA Area Shown Test 4 Jan. 2016 Test 5 Sept. 2016 Test 1 Oct. 2006 Test 2 May 2009 Test 3 Feb. 2013 MT. MANTAP NORTH TUNNEL ENTRANCE DEBRIS PILE The underground tunnels culminate in a hook, allowing them to seal themselves after the blast. This design is believed to have come from Pakistani nuclear test sites. 0.5 miles Satellite Image by CNES / Astrium via Google Earth | Magnitude data from the Nuclear Threat Initiative and USGS Note: Tunnel system based upon an analysis by Frank Pabian and Siegfried Hecker. NORTH KOREA Area Shown Test 5 Magnitude 5.3 Sept. 2016 Test 4 Magnitude 4.8 Jan. 2016 MT. MANTAP Test 2 Magnitude 4.5 May 2009 Test 1 Magnitude 4.1 Oct. 2006 Test 3 Magnitude 4.9 Feb. 2013 NORTH TUNNEL ENTRANCE DEBRIS PILE EAST TUNNEL ENTRANCE The underground tunnels culminate in a hook, allowing them to seal themselves after the blast. This design is believed to have come from Pakistani nuclear test sites. 0.5 miles Satellite Image by CNES / Astrium via Google Earth | Magnitude data from the Nuclear Threat Initiative and USGS Note: Tunnel system based upon an analysis by Frank Pabian and Siegfried Hecker. NORTH KOREA Area Shown Test 4 Jan. 2016 Test 5 Sept. 2016 Test 2 May 2009 Test 1 Oct. 2006 Test 3 Feb. 2013 MT. MANTAP NORTH TUNNEL ENTRANCE DEBRIS PILE The underground tunnels culminate in a hook, allowing them to seal themselves after the blast. This design is believed to have come from Pakistani nuclear test sites. 0.5 miles Satellite Image by CNES / Astrium via Google Earth | Magnitude data from the Nuclear Threat Initiative and USGS Note: Tunnel system based upon an analysis by Frank Pabian and Siegfried Hecker.

Experts say the North’s tunnel system will quite likely play a significant role in future testing because it is the busiest area at the Punggye-ri nuclear test site, has a conspicuous perimeter fence and has the largest amount of protective rock directly overhead.

Several independent teams, including ones from China, South Korea, Norway and the United States, have gathered seismic readings from the North’s bomb tests. In addition, a world body known as the the Comprehensive Test Ban Treaty Organization, based in Vienna, Austria, operates a sprawling network of global sensors.

Last year, a team of South Korean scientists confirmed a test’s exact location within Mount Mantap by studying data from a radar satellite that detected subtle elevation changes on the mountain’s surface.

So far, North Korea’s nuclear tests have grown more destructive.

Circles sized by destructive power 35 17 17 8 2 kilotons 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Circles sized by destructive power 35 17 17 8 2 kilotons 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Circles sized by destructive power 35 17 17 8 2 kilotons Oct. 2006 May 2009 Feb. 2013 Jan. 2016 Circles sized by destructive power 35 17 17 8 2 kilotons Oct. 2006 May 2009 Feb. 2013 Jan. 2016 Source: Nuclear Threat Initiative

Note: Destructive power numbers represent worst case estimates.

The North has shown technical savvy in pacing its nuclear tests to increase the amount of time for bomb makers to conduct detailed analyses of the blasts and learn from mistakes.

“They’ve done five tests in 10 years,” said Siegfried S. Hecker, a Stanford professor who once directed the Los Alamos weapons laboratory in New Mexico, the birthplace of the atomic bomb. “You can learn a lot in that time.”

In contrast, he added, India and Pakistan conducted a rush of nuclear detonations in May 1998 in what experts called a blitz of saber rattling. “They couldn’t have learned much,” Dr. Hecker said.

North Korea may be focused on increasing the power and range of its nuclear weapons.

North Korea’s past tests are thought to have centered on mastering a simple type of atomic bomb, known as an implosion device.

Some of the tests may have featured “boosted” atomic bombs, however — meaning that an injection of tritium, a radioactive form of hydrogen, could have increased their destructive power. All such fuels, known as thermonuclear, need the high heats from an exploding atom bomb for ignition.

“It’s possible that North Korea has already boosted,” said Gregory S. Jones, a scientist at the Rand Corporation who analyzes nuclear issues. Like other experts, he pointed to the nation’s two nuclear detonations last year as possible tests of small boosted arms.

As signs of the North’s interest in boosting, experts cite modifications to a reactor that could make tritium, as well as construction of a plant that could gather up the radioactive gas. Boosted arms can raise the destructive power of atomic blasts or greatly reduce their need for atomic fuel. That savings can significantly lighten and shrink the resulting arms, making them easier to hurl over long distances. Boosting is considered a main step to scaling down warheads so they can fit atop intercontinental missiles.

Nine countries possess nuclear arms, most having advanced over time from making simple atom bombs to advanced hydrogen bombs. Nuclear experts say North Korea’s program is at an intermediate phase of development — somewhere between stages one and three. The secret to achieving more destructive power is to raise the amount of thermonuclear fuel that an exploding atom bomb can ignite.

STAGE 1

Implosion Atom Bomb

uses conventional explosives to compress and ignite atomic fuel Destructive power



1

Hiroshima Explosive layer Atomic fuel Explosive layer Atomic fuel Explosive layer Atomic fuel Explosive layer Atomic fuel Explosive layer Atomic fuel STAGE 2

Boosted Atom Bomb

uses a bit of thermonuclear fuel inside the atomic core



3

Hiroshimas Thermonuclear gas Thermonuclear gas Thermonuclear gas Thermonuclear gas Thermonuclear gas STAGE 3

Layered Atom Bomb

uses more thermonuclear fuel outside the atomic core



25

Hiroshimas Solid thermonuclear fuel Solid thermonuclear fuel Solid thermonuclear fuel Solid thermonuclear fuel Solid thermonuclear fuel STAGE 4

Hydrogen Bomb

uses lots of hydrogen fuel that the nearby atom core ignites



1,000

Hiroshimas Atom bomb Solid thermonuclear fuel Atom bomb Solid thermonuclear fuel Atom bomb Solid thermonuclear fuel Atom bomb Solid thermonuclear fuel Atom bomb Solid thermonuclear fuel

Note: Destructive power for each stage is based on early tests in the US and USSR, not on current stockpiles. Note: Destructive power for each stage is based on early tests in the US and USSR, not on current stockpiles. Note: Destructive power for each stage is based on early tests in the US and USSR, not on current stockpiles. Note: Destructive power for each stage is based on early tests in the US and USSR, not on current stockpiles. Note: Destructive power for each stage is based on early tests in the US and USSR, not on current stockpiles.

Experts say the likelihood of North Korea making strides in its nuclear program has risen with the recent evidence that the nation tried to sell excess lithium-6, which is the main ingredient for making thermonuclear fuels, including tritium.

So too, satellite images of the mountainous test site reveal the digging of the deep tunnel system, which could allow the detonation of a larger device.

A gathering at the remote site is considered a strong sign.

On March 28, a satellite image showed people gathered in front of an administrative building at the test site. The last time such a large group was observed was on Jan. 4, 2013, a little more than a month before North Korea’s third nuclear detonation.

March 28, 2017 Jan. 4, 2013 People in parade formation People gathered in formation March 28, 2017 Jan. 4, 2013 People in parade formation People gathered in formation March 28, 2017 Jan. 4, 2013 People in parade formation People gathered in formation March 28, 2017 Jan. 4, 2013 People in parade formation People gathered in formation Source: DigitalGlobe Imagery

Experts see the recent gathering as yet another sign that the North may be getting closer to detonating a device in its increasingly long series of nuclear tests.