AS NEW year fireworks crackle and boom amid the neon beside the Yalu river, a small boat inches silently across the stretch of water that forms the border with North Korea. 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 (see map). “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.

Fuel, rice, wheat and basic consumer goods all flow legally, usually by lorry over bridges on the Yalu, into North Korea. Imports from the North include minerals, coal, scrap metal and seafood. There is also a thriving black-market trade both ways, usually by boat. This feeds the growing demand for other non-staple products among the new North Korean nouveaux riches. Border police, especially in the North, are known to take bribes to allow illicit trade to pass. One illegal North Korean export causing social problems is crystal meth, a drug known in China as bingdu, or “ice”.