The coal mines, with their steep slag heaps that jut out like dark pyramids on the horizon, have been a defining feature of eastern Ukraine for more than a century. They supplied the Russian Empire, and then the Soviet Union, with much of its raw energy. But the collapse of the USSR in 1991, the past eight years of separatist conflict with pro-Russian militias and the Kremlin’s new offensive have meant that most of the mines have been closed.