Estimates for recoverable shale gas just keep falling. Last year, the Potential Gas Committee, an industry consortium that focuses on long-term projections, estimated that recoverable natural gas from shale deposits in the United States would amount to 687 trillion cubic feet (tcf). (This optimistic appraisal laid the groundwork for the oft-repeated notion that the United States has 100 years of natural gas supply at current rates of consumption. The estimate was also based on so-called "speculative resources" of another 615 tcf.)