Strip mining is a form of surface mining which involves coal excavation, this form of mining accounts for 40% of the world's coal mines. The rock and dirt covering the ore is removed one strip at a time. Once the ore has been taken from the first strip, a second strip is cut next to it, and the waste from the second strip is used to back fill the first. This goes on until the ore is exhausted, and then the waste from the first strip is used to fill in the last.