HONG KONG • When North Korea unleashed an earth-shaking nuclear bomb of up to 250 kilotonnes on Sept 3, one man was not fazed.
About 9,650km from Pyongyang, in Colorado, Mr Joseph Bermudez Jr has for years watched the isolated regime's nuclear testing facility, peering at blobs and shadows on high-resolution satellite images.
Slight changes - movements of vehicles, equipment and people at the mountain fortress - matter. From February, he and his colleagues spotted activity at one of the three main tunnels leading to underground testing bunkers.
"We knew at the beginning of the year that they would test," said Mr Bermudez, who writes assessments for the Johns Hopkins School of Advanced International Studies' North Korea website, 38 North. "So when that day came, in fact my reaction was, 'oh, they finally tested'."
That blast, the regime's most powerful detonation to date, was another step in its efforts to acquire the ability to hit the continental US with a nuclear-tipped warhead. Alongside the atomic programme, leader Kim Jong Un has this year launched a succession of ballistic missiles, each flying further or higher than the last.
Mr Kim's actions have also set off a tense war of words with US President Donald Trump, who is exasperated by the regime's dogged pursuit of nuclear weapons in the face of global sanctions and condemnation. With Mr Trump threatening to attack North Korea if provoked, parsing the regime's actions has become ever-more important.
Mr Bermudez is one of a handful of analysts in the United States and Asia who peer into Mr Kim's backyard on a daily basis, poring over commercial satellite images and other data. In such a volatile environment, their assessments can help temper fears or keep in check speculation sparked by the North's propaganda machinery, with its fiery pledges to annihilate the US.
"Typically what we've identified is that prior to a test, they conduct some additional excavation in the tunnels," said Mr Bermudez. "They move more equipment in and you see more people moving around."
In experts' crosshairs is Punggye-ri, an active test site built in a secluded mountain valley north-east of Pyongyang. The area, the site for all six of North Korea's nuclear blasts, has a "virtually infinite amount of space" for underground testing and its granite bedrock is ideal for containing large explosions, said director Jeffrey Lewis at the East Asia Non-proliferation Programme of the Middlebury Institute of International Studies in Monterey, California.
Examined over time, a picture emerges of what normal or suspicious activity at Punggye-ri looks like. One ominous sign is when the bustle grinds to a halt: Vehicles, troops and workers withdraw and the site appears "tidier".
"That is often an indicator that things have been buttoned down and they're ready to do something," said 38 North defence technology expert Jack Liu.
Mr Lewis said the takeaway is Punggye-ri is constantly being prepared for tests.
Given North Korea's firing of another intercontinental ballistic missile on Nov 29, Mr Lewis said the entire US is already in range.
"That missile went high enough and far enough that, if it had been fired at the US it could have hit Mar-a-Lago," Mr Trump's Florida-based resort, he added.
"It's too late to stop the programme," he said. "The time to stop the programme was 10 to 15 years ago. They're going to have thermonuclear weapons on an ICBM that can target the US."