Hydraulic fracturing, or fracking, is a drilling method used to extract petroleum (oil) or natural gas from deep in the Earth. In the fracking process, cracks in and below the Earth's surface are opened and widened by injecting water, chemicals, and sand at high pressure. Some resources extracted through fracking are called “tight oil” or “tight gas,” because these pockets of fossil fuels are tightly trapped in hard shale rock formations.

Source: National Geographic