Hydraulic fracturing. Artwork of rock layers being fractured by fluids pumped in from the surface, a process known as fracking or hydraulic fracturing. It is usually used to extract gas from shale deposits. A mixture of sand, chemicals and water is pumped under high pressure through horizontal wells (lower right) to form fissures in the shale. The sand keeps the fissures open, allowing the gas to flow into the well and be taken to the surface. Fracking is known to cause small earthquakes (microseismic events). Also shown here is a water table, with a wind-driven pump and borehole (upper right) being used to raise water to the surface.