Breaking rock by water pressure
Millions of gallons of water, sand and lubricating chemicals are forced at high-pressure into bore holes sometimes 3km underground, which shatters the surrounding rocks and allows the gas to flow out and back up the bore hole.
However after its use to break the rock, the fracking mixture has to go somewhere. It can only either push back out to the surface along with the gas, or remain in the rock formation in the spaces created by the fracturing process and where the resulting gas has leaked out.
The sand stays underground, holding open the cracks formed in the rock and filling the voids where the gas was found. Some, but not all of the water comes back to the surface as a thick sludge, which is stored in ponds, before it is disposed of. The sludge not only contains the polluting fracking chemicals, but also low level radioactive material that is naturally found in the rocks.