1. Operated by, moved by, or employing water or other liquids in motion.
2. Operated by the pressure created by forcing water, oil, or another liquid through a comparatively narrow pipe or orifice.
3. Of or pertaining to water or other liquids in motion.
4. A reference to hydraulics.
5. Hardening under water; such as, a cement.

A process used in nine out of ten natural gas wells in the United States, where millions of gallons of water, sand, and chemicals are pumped underground to break apart the slate rock and release gas: "The explosive growth of hydraulic fracking; especially, in areas where gas is often located closer to people’s homes and population centers has exposed some of the environmental risks associated with shale gas."

"Scientists and environmentalists are worried that the chemicals used in hydraulic fracturing may pose a threat either underground or when waste fluids are handled and sometimes spilled on the surface of the ground."

"Hydraulic frackings involve the injection of more than a million gallons of water, sand, and chemicals at high pressure down and across into the horizontally drilled wells as far as 10,000 feet below the surface of the earth."

"The pressurized mixtures used in hydraulic frackings cause the rock layer of shale to crack and these fissures are held open by the sand particles so the natural gas from the shale can flow up the well to the surface and into storage tanks from where it is taken for distribution and sold to users."

The wings of a fracture extend away from the wellbore (any hole drilled for the purpose of exploration or extraction of natural resources) in opposite directions depending on the natural stresses within the shale formation: "Hydro fracking is a process used to release petroleum, natural gas, coal seam gas, or other substances for extraction, via a technique called induced hydraulic fracturing."

"The process of hydraulic fracturing, or fracking, involves drilling a hole deep into the dense shale rocks which contain natural gas, and then pumping in at very high pressure vast quantities of water mixed with sand and chemicals."

"Hundreds of chemicals are used, some of them potentially toxic, including methanol, naphthalene, benzene, and lead."

"Although hydraulic fracturing has been done since the 1940's, the rapid expansion of its application to unconventional gas production has raised environmental concerns around the world."

"The process of hydro fracking involves water availability, its use and potential contamination, and on greenhouse-gas emissions from shale gas production."

"Many shale deposits are buried under aquifers, and if the cement casing around the wellhole is not adequate, then the process of drilling and fracking can release the chemicals into the aquifer and such leaks of methane can take place, possibly leading to fires or explosions."

"Natural gas is located in densely packed rock; such as, shale and the hydraulic fracturing starts with a hole that is drilled into the shale rock layer, which is not nearly as thick as it is long."

"As soon as the drill reaches the layer of shale rock, it makes a right angle turn and moves horizontally and then the well bore is lined with concrete so it will keep fluids from seeping out and a water and sand mixture is pumped under high pressure down the well bore and into the holes where it fractures the shale."

"Gas in the rock is forced into the well bore and a temporary plug is inserted; and finally, the plugs are drilled out and the gas flows to the surface."

"To summarize, the U.S. federal government is beginning to undertake a review of the chemicals that are used in hydraulic fracturing, the relatively recent technique used to release vast stores of natural gas buried underground."

"The technique uses millions of liters, or gallons, of water, sand, and chemicals; including carcinogens; such as, benzene which is injected at high pressure to fracture the rock and release the stored gas."

"Environmental groups have exposed several persistent dangers: leaks in wells because of faulty casing or migration through layers or rock; breaches in the above-ground tanks that are meant to store used drilling chemicals; and an increase in air emissions."