For natural gas storage facilities fall into several categories. Seasonal storage comprises depleted gas fields; Aquifers; Salt Cavity Storage; mined caverns; and disused mines. Peak storage includes Gas Holders, Line Pack, lengths of pipeline buried specifically for storage use, and LNG storage used either for base-load or peak-shaving duties, depending on the market. Increasingly used in liberalised markets to enable gas to be trade at any time of the year for reasons not related to peak demand. See also Peak Shaving.