Complex sentence: Muhlenberg County has been a major coal-producing region for the United States for many years; during most of the 1970s, Muhlenberg County annually produced more coal than anywhere else in the world.
Split into simpler sentences: Muhlenberg County has been a major coal-producing region for the United States for many years. In fact, during most of the 1970s Muhlenberg County annually produced more coal than anywhere else in the world.