This groovy little video does a good job of explaining what is going on during hydraulic fracturing and why it’s such a concern to so many.
Hydraulic fracturing, or fracking, is a process for extracting hydrocarbons. Fracking is done after the drilling is complete. Water and chemicals are injected into the earth, fracturing the rock and releasing the trapped hydrocarbons. In many places, only 10% of the fracking fluid returns to the surface. That means that 90% of the chemical-filled water is still down there, percolating up through the rock and potentially into aquifers and the drinking water in homes.
Because fracking is exempt from regulation under the Clean Water Act, large-scale studies to determine the exact effects of the procedure have not been funded. Small studies have been done and have connected fracking to earthquakes.