Abstract: After they have been washed out, cavities (4) in rock salt contain a large quantity of brine (8) at a temperature which is lower than the temperature of the surrounding formation (2). This means that the cavity cannot be sealed quickly because the brine will expand progressively as it heats up. The method of the invention serves to shorten the time period between the end of working the cavity for salt and being able to abandon the cavity finally from about 30 years to about 2 years. The method consists in injecting quantities of a mixture (12) into the cavity (4), the mixture being of greater density than brine (8) and being capable of setting, with equivalent quantities of brine (8) being returned to the surface, and then in keeping the cavity (4) open after it has been completely filled with mixture (12) for as long as it takes the rock salt (2) to creep and fill up the shrinkage voids which appear during setting of the mixture (12), after which the cavity (4) can be sealed.