Caverns are leached in salt formations by injecting fresh water into a bore hole. The salt is dissolved and produced back to the surface. The principle is shown in the figure below. Fresh water can be injected directly through an inner pipe and the brine is then being produced through the annulus of the leaching string. To avoid uncontrolled leaching of the cavern roof a blanket is applied on top which consists usually of leachig-inert oil or gas.  
        
The leaching process can be controlled by the depth of the pipes, the leacing rate and the blanket. 

Cavern leaching

Position of caverns in a salt dome

( Picture courtesy DEEP.KBB)