New applications of horizontal drilling techniques and hydraulic fracturing—in which water, sand, and chemical additives are injected under high pressure to create and maintain fractures in underground formations— allow oil and natural gas from shale formations (known as “shale oil” and “shale gas”) to be developed. As exploration and development of shale oil and gas have increased––including in areas of the country without a history of oil and natural gas development–– questions have been raised about the estimates of the size of these resources, as well as the processes used to extract them.