Cutaway artwork of rock layers being fractured (lower centre) by fluids pumped in from the surface, a process known as fracking or hydraulic fracturing. It is usually used to extract natural gas from shale deposits. A mixture of sand, chemicals and water is pumped under high pressure through horizontal wells to form fissures in the shale. The sand keeps the fissures open, allowing the gas to flow into the well (inset circle) and be taken to the surface. Fracking can cause small earthquakes (microseismic events), and there are concerns about the potential effect on the water table (blue layer, centre). A well drawing water from the water table is at upper right.