NOVELTY - Making an underground cavern includes drilling a well into a subterranean formation comprising carbonate rock, pumping low-viscosity aqueous acid solution through the well, ejecting aqueous acid solution from the well to contact carbonate rock, reacting the aqueous acid with the carbonate rock, and removing the reaction products from the subterranean formation to form a cavern defined by the remaining subterranean formation.    USE - The method is used for making underground cavern used for storage of fluid, e.g. natural gas, hazardous waste fluid, compressed air, or petroleum. It may be used for producing calcium chloride. (All claimed)    ADVANTAGE - The resulting cavern enables safe and efficient storage of large volumes of liquids and gases.    DETAILED DESCRIPTION - Making an underground cavern includes drilling a first well into a subterranean formation comprising carbonate rock, pumping a low-viscosity aqueous acid solution through the first well, ejecting the aqueous acid solution from the first well to contact carbonate rock of the formation, reacting the aqueous acid with the carbonate rock to form reaction products comprising salt in aqueous solution and carbonic acid in equilibrium with carbon dioxide, and removing the reaction products from the subterranean formation to form a cavern defined by the remaining subterranean formation.    INDEPENDENT CLAIMS are also included for:    (1) a manufactured cavern comprising a single open void defined by a subterranean formation comprising carbonate rock, and a fabricated access between the cavern and the earth surface for providing direct access to the cavern by people, where the cavern is at a predetermined depth and formed by acid dissolution of carbonate rock of the formation; and    (2) storage of fluid, e.g. natural gas, hazardous waste fluid, or compressed air, by making an underground cavern according to the above method, pumping fluid into the cavern, and storing an amount of the fluid (e.g. greater than or equal to 0.1 billion cubic feet natural gas as measured at standard temperature and pressure or zero depth conditions) in the cavern, where the fluid is liquid at standard temperature and pressure.    DESCRIPTION OF DRAWING(S) - The figure schematically illustrates a one-well design.