Fracking perforating gun. Close-up of the holes in a perforating gun after use. These devices are explosive charges inside a specialized pipe. They are inserted deep into a oil or gas well to make perforations through the casing and surrounding cement, into the rock. This allows oil or gas to flow. In fracking, these initial rock fractures allow hydraulic fracturing fluid to be pumped into the rock formation, creating many new fractures. Fracking or hydraulic fracturing is a drilling technique used to extract gas from shale deposits. Sand in the fracking mixture keeps the fissures open, allowing gas to flow into the well and rise to the surface. Photographed in Arkansas, USA.