North Korea’s coal exports remained relatively constant over the course of 2014, and were North Korea’s most valuable export between January and November last year, according to official data from Chinese customs.

In terms of volumes, North Korea shipped on average 1.2 million tons a month, generating over a billion U.S. dollars over the course of 2014 and nearly $5 billion since 2011.

“I think coal is their most valuable single export commodity to China – at least so far as the customs statistics tell us” David Von Hippel, a senior associate at the Nautilus Institute for Security