Across the prairies of Wyoming, past buffalo herds and miles from any town lies the world's biggest coal mine. For 24 hours a day, seven days a week, the North Antelope Rochelle Mine is at work. Using hulking trucks, workers strip off hundreds of feet of grass, soil and rock to get to the coal below.

They blast the coal face into chunks that are crushed and taken by train to 60 power plants on the US's busiest railway. Demand is so high for coal, which fuels 50 per cent of US electricity output, that the process never stops.