Abstract: A method of putting under gas a saline cavity 3 for storing a gaseous substance, the cavity being formed by being washed out in rock salt and being filled with brine 5 once the washing-out step has been completed, the cavity includes a bottom 31 and a roof 30, a well 1 drilled for washing-out purposes opening out in the roof and putting the saline cavity into communication with the surface, the well also serving as an injection well for inserting the gaseous substance to be stored. The method includes a step of drilling an associated dewatering well 4 from the surface in the proximity of the injection well, the dewatering well meeting the saline cavity close to the bottom thereof so as to dewater the brine from the cavity through said dewatering well as the gaseous substance is injected into the cavity via the injection well.