Since the 1920s, phosphorite mining has polluted the air and water of Estonia. The former Soviet Union republic is rich in phosphorite deposits, which can be used to make phosphorus fertilizers. In the 1960s, the Soviet Union began exploiting Estonia’s deposits with large-scale mining operations. The ensuing problems were caused not by the phosphorite, but by the layers of oil shale that were removed in the process of extraction. The excess shale was typically dumped close to the mine, where it would continually catch fire and pollute the groundwater.