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. 

• If there is underground water nearby, there is a risk that carcinogenic (cancer causing) chemicals can  leak from the shattered rocks into the water table, polluting local water supplies and aquifers. 
• If the ponds leak, surrounding soils can be badly polluted and the same chemicals can potentially leak downwards into the water table.
• Natural Gas is methane – a very polluting greenhouse gas.  Not all of the gas escapes up the borehole and some can leak out into the atmosphere. 
• Minor earthquakes have been triggered by the movements of rocks

By opening up new sources of natural gas, fracking promises to greatly increase the availability and reduce the cost of gas. Some argue that this is a good thing, as it can be used to replace coal in power stations, where it is much less polluting and releases much less CO2. However the risk is that it allows us to continue to burn fossil fuels cheaply, and takes attention and investment away from developing renewable energy sources to replace them.