materials. For example, alongside livestock lots, spoil banks are formed of manure and other slurry periodically removed from the surface of the livestock lot areas. Environmental effects Spoil tips sometimes increased to millions of tons, and, having been abandoned, remain as huge piles today. They trap solar heat, making it difficult (although not impossible) for vegetation to take root; this encourages erosion and creates dangerous, unstable slopes. Existing techniques for regreening spoil tips include the use of geotextiles to control erosion as the site is resoiled and simple vegetation such as grass is seeded on the slope. The piles also create acid rock drainage, which pollutes streams and rivers. Environmental problems have included surface runoff of silt, and leaching of noxious chemical compounds from spoil banks exposed to weathering. These cause contamination of ground water, and other problems. In the United States, current state and federal mining regulations require that the earth materials from excavations be removed in such a fashion that they can be replaced after the mining operations cease in a process called mine reclamation, with oversight of mining corporations. This requires adequate reserves of monetary bonds to guarantee a completion of the reclamation process when mining becomes unprofitable or stops. (See for example, the Surface Mining Control and Reclamation Act of 1977.) Subterranean combustion In some spoil tips, the waste resulting from industries such as coal or oil shale production can contain a relatively high proportion of hydrocarbons or coal dust. Spontaneous subterranean combustion may result, which can be followed by surface fires. In some coal mining districts, such fires were considered normal and no attempt was made to extinguish them. Such fires can follow slow combustion of residual hydrocarbons. Their extinction can require complete encasement, which can prove impossible for technical and financial reasons. Sprinkling is generally ineffective and injecting water under pressure counter-productive, because it carries oxygen, bringing the risk of explosion. The perceived weak environmental and public health effects of these fires leads generally to waiting for their natural extinction, which can take a number of decades. Landslides The problem of landslides in spoil tips was first brought to public attention in October 1966 when a spoil tip at Aberfan in Wales gave way, killing 144 people, 116 of them children. The tip was built over a spring, increasing its instability, and its height exceeded guidelines. Water from heavy rainfall had built up inside