Salt leached caverns

Salt may occur underground in extensive thick strata or in domes.

It is easily extracted by dissolving it in water; in this way, huge artificial underground caverns can be created, situated at a depth typically ranging between 200 and 2000 metres.

The caverns are solution mined.  Fresh or low salinity water is injected below ground through a borehole and progressively dissolves the salt in a controlled manner (leaching). Traditionally, the resulting brine is removed through the same hole, but today, two separated drillings are often used. 

The caverns are usually elongated in shape - several hundred metres in height and several tens of metres in diameter - and may have a volume of several hundred thousand cubic metres.  Their shape and volume are determined by sonar.

Cavern stability is monitored by periodic sonar checks of its geometry and continuous surveillance of acoustic emissions (seismic monitoring techniques).

Its tightness is guaranteed by the salt properties.