Geothermal energy in the United States Geothermal Data System (NGDS). Through the NGDS, many older paper archives and drill logs that are stored at state geological surveys are now being digitized and made available for free to the public. The underground hot water and steam used to generate geothermal power may contain chemicals that could pollute the air and water if released at the surface. Hydrogen sulfide, which is toxic in high concentrations, is sometimes found in geothermal systems. Newer methods of generating geothermal power separate the hot steam collected underground from the steam used to power turbines, and substantially reduce the risk of releasing air-polluting
Underground mining (soft rock) Underground mining (soft rock) Underground soft rock mining is a group of underground mining techniques used to extract coal, oil shale, potash and other minerals or geological materials from sedimentary ("soft") rocks. Because deposits in sedimentary rocks are commonly layered and relatively less hard, the mining methods used differ from those used to mine deposits in igneous or metamorphic rocks ("see Underground mining (hard rock)"). Underground mining techniques also differ greatly from those of surface mining. The number sign, or hash sign (#) is often used as shorthand to denote shaft or seam, as in 4# (4 shaft or seam