The coal mines in the PRB advance quickly enough to see a lot of change from year to year. Two animations available here show the annual change of the mines in the regions.
The first animation shows almost one image per year back to 1984 of both the Black Thunder and North Antelope Rochelle Complex Mines. The second one listed is a close-up of the North Antelope Rochelle Complex and covers the same time period.
The date of each image appears in the lower right corner.