Ore mills generate large amounts of waste, called tailings. Hematite is present in the waste tailings of iron mines. Mining processes produce an excess of waste materials known as tailings. Thickeners are used for dewatering slurries of either tailings or product. Large earthen dams may be constructed and then filled with the tailings. Pyrite oxidation is often the cause of coal spontaneous ignition in old mine tailings. Structures such as levees, dikes and tailings dams are not included in the lists. Canadian company Kovit Engineering Limited specialized in solutions for treating the tailings from ore enrichment. Penstocks are also used in mine tailings dam construction. However, the greatest danger of tailings ponds is dam failure. By 2009, Suncor was working to reduce the amount of bitumen entering tailings ponds. The dam failure released around 12 million cubic meters of tailings. Overfishing and pollution from gold mine tailings resulted in the decline of the fishery. Consequently, there is little surface disturbance and no tailings or waste rock generated. Paste tailings is a modification to the conventional methods of disposal of tailings (pond storage).