Natural gas can also exist in Devonian shale deposits. Devonian shales are formed from the mud of shallow seas that existed about 350 million years ago (during the Devonian period of the Paleozoic era). These shales can contain natural gas, usually when two thick, black shale deposits ‘sandwich’ a thinner area of shale. Because of some of the properties of these shales, the extraction of natural gas from shale formations is more difficult (and thus expensive) than extraction of conventional natural gas. Most of the natural gas containing Devonian shale in the U.S. is located around the Appalachian Basin. Although estimates of the amount of natural gas contained in these shales are high, it is expected that only about 10 percent of the gas is recoverable. However, their potential as a natural gas supply is still very promising, given an adequate technological and economic environment. The EIA estimates that there are 55.42 Tcf of technically recoverable shale gas in the United States, representing just under 5 percent of total recoverable resources