Research has linked an earthquake (magnitude 3.9) that struck near Youngstow in Ohio, USA on 31 December 2011 to the underground injection of wastewater from fracking.
The practice of hydraulic fracturing, or fracking, involves injecting water, sand and other materials under high pressures into a well to fracture rock. This opens up fissures that help oil and natural gas flow out more freely. This process generates wastewater that is often pumped underground as well, in order to get rid of it.