This invention relates to a new and novel method for the underground storage of fluid materials including those which are readily soluable or reactive with water or brine such as concentrated caustic soda, ethylene dichloride or anhydrous ammonia within chambers developed in salt formations via solution mining and from which essentially all the brine has been removed; and the chambers thereby produced. More particularly this invention relates to methods for making underground storage chambers; of recovering the brine formed in the making of the chamber; isolating the contaminants, i.e., the insolubles mixed with brine resulting from the solution mining of the storage chamber, such that fluids soluable or reactive with water or brine can be stored therein; and also to a method for controlling the velocity of free fall of materials injected into the chamber for storage, such that erosion of the equipment employed is minimized.