Mines frequently have been used by twentieth century microbiologists as a source of samples in the study of the microbial ecology of the geologic formations that the mines penetrate. These investigations can be classified, based upon the goals of the original researchers, into three categories. The earliest studies utilized mines to acquire samples of microorganisms thought to be responsible for the formation of, or for detrimentally affecting, economic minerals and/or oil and gas deposits. Those 46studies, related to mineral deposits, often focused on the role of microorganisms in acid mine drainage. The second type of investigation focused on the potential affects of allochthonous (foreign or introduced) and autochthonous (indigenous) microorganisms on the reliability of potential geologic repositories for the storage of highlevel nuclear waste (see Chapter 15). The third category of research utilized mines to obtain samples of indigenous endolithic microorganisms to assess the possibility of anabiosis or dormancy, and more recently, to characterize the presence, abundance, and metabolic capabilities of deep subsurface microorganisms for the purpose of in situ bioremediation.