One year ago, a French frigate in the East China Sea captured images of a North Korea-linked oil transfer between two ships in the dead of night. The photos show two oilers side-by-side on still, pitch-black waters, with thick tethers drooping between them. Deckhands worked under the glare of pale-green flood lights to fasten a heavy oil line between the two and quickly start pumping before daybreak.

Such nighttime transfers are a tried-and-true tactic for Kim Jong Un’s oil smugglers, who understand that most satellites whizzing by overhead can’t see them at night.