Complex sentence: The historical ratio of underground mining to open-cast mining is approximately 50-50, although it is tending to move away from open-cast production as bed depths increase--exhausted open-cast areas are gradually being regenerated through land recultivation and reforestation.
Split into simpler sentences: The historical ratio of underground mining to open-cast mining is approximately fifty-fifty, although it is tending to move away from open-cast production as bed depths increase. Exhausted open-cast areas are gradually being regenerated through land recultivation and reforestation.