When North Korea unleashed an earth-shaking nuclear bomb of as much as 250 kilotons on Sept. 3, one man wasn't fazed.

Almost 9,650 km (6,000 miles) from Pyongyang, in Colorado, Joseph S. 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, Bermudez and his colleagues spotted activity at one of the three main tunnels leading to underground testing bunkers.