Answer: The coal, oil and natural gas we are consuming now are the remnants of plants, trees and animals that were living hundreds of millions of years ago. There is no way to replace them, as conditions on Earth are much different now.
Explanation: Mines and wells are being depleted much faster than they were a hundred years ago when demand was much less-- fewer people, fewer cars, fewer planes. . . and people believed there was an endless supply. Many of those mines and oil wells are now empty.