Fracking has mostly been studied because of what it can potentially do to the surface environment. The chemicals used in fracking fluid, as well as the gas and brines that can come back up the wells, all pose environmental risks that have to be managed. What’s often not considered is that the well is a bit like a two-way street. The fracking fluid, which is anything but sterile when injected, also contaminates the environment deep under the Earth’s crust.