Tailings are a type of rock waste from the mining industry. When a mineral product is mined, the valuable portion is usually embedded in a rock matrix called ore. Once the ore has been stripped of its valuable minerals, sometimes through the addition of chemicals, it is piled up into tailings. Tailings can reach immense proportions, appearing in the form of large hills (or sometimes ponds) on the landscape.
Some mining wastes become very fine after they have been ground up during processing. The fine particles are then generally mixed with water and piped into impoundments as a slurry or sludge. This method cuts down on dust problems, and at least in theory, the impoundments are engineered to let excess water flow out without leaking tailings. Coal ash, while not a type of tailing, is a coal burning by-product stored the same way, and carrying similar environmental risks.