[Catfish]

Fracking is a two-sided sword. Usually this is only done when the oil/gas will not flow freely anymore (often when the oil well is getting exhausted, pressure drop etc.), it can be used for oil and gas reservoirs.

Those reservoirs are not big caves, but the stuff we want is distributed in cracks and geologic faults, gas like oil. The reservoir is located in stone or sediment, with water, oil and gas filling up the spaces between e.g. sand corns or in smallish cracks. You have a porosity, and a permeability.
To get more out you try to increase the permeability in the reservoir.

To squeeze more out you use what is called hydraulic fracking, you press a fluid down the production pipe, causing the sediment to be "blown up", and widen cracks and pore space, to make the rests of hydrocarbons flow freely again.
The fluid used is mostly loaded with chemical drag reducers like detergents, but also with specially formed plastic, stone or ceramic particles, to keep those opened cracks open, when the remaining hydrocarbons get out. Some of this stuff is toxic.

So the geologic layers being pumped up increase in volume, and decrease again, which of course has an impact on the surface since the whole reservoir structure is "inhaling" and "exhaling" in a way.
Further problems are the chemicals used because regardless how careful you are, a lot of this stuff cannot be regained at the surface, additionally the production pipes of older wells can be badly corroded, and leaking.
So you also lose some of the stuff to groundwater horizons. I guess you can imagine what can happen next, from gas to all kinds of chemical pollution, in the drinking water.

The whole procedure is also expensive, financially.