A low block of flats is just visible on the other side, pitch-black save for a single prick of electricity. Clever timing by the smugglers, smiles a watching Chinese resident: "All the police are eating their new year meal."

When North Korea tested a third nuclear weapon on February 12th, it was not security that was troubling the residents of towns along China’s 1,420km (880-mile) border with North Korea. "Trade [with North Korea] is a large part of Dandong’s economy," says Wu Yang, owner of an ornament company that uses semi-precious stones from the North. "I’m worried it will be affected," she says.