Salt in the chemical sense is an ionic compound of metals with non-metals. Another definition is, its an ionic compound that can be formed by the neutralization reaction of an acid and a base. The group of salts is quite big and includes limestone and gypsum. But what we are meaning here is the salt in salt water, those ionic compounds which are easily dissolved by water and which you can get by evaporating sea water in a bowl. During Earth's history numerous times geographic conditions existed which created huge depressions with sporadic inflow of sea water. The result are massive deposits of salt.
The main mineral of the so called salt rocks, is the well known table salt which is used in the kitchen. The chemical name is sodium chloride (NaCl), as a mineral it is called halite. But the rock also contains various other salt varieties with similar chemical characteristic. Some are used as mineral fertilizer, others are unhealthy.
Salt caves are an extremely rare variety of karst caves. They are formed by the dissolution of salt by rain water. This seems quite simple and easy, and actually salt caves are the ones which are formed quite fast. Salt is so easily dissolved by water, any occurrence of salt gets washed away very fast. Much faster than the movements of the crust exposing the salt. Salt normally does not even reach the surface in areas with humid climates (high precipitation). Normally the salt is dissolved by the ground water which causes salt water springs. Such springs were used by man for thousands of years to produce salt.
The main prerequisite of the formation of salt caves are salt outcrops at the surface, which atually restricts them to arid (desert) climates. Even in arid climates with their low precipitation, there are some rare events with rain. At this events the rain water forms huge and wild rivers running down the wadis, the desert valleys. This water forms solution caves in salt deposits. This process is so fast, salt caves change considerably with each rainfall. And despite the rareness of rainfalls, the life of salt caves is very short. The life of a salt cave may take several thousand years or only some years.
Salt caves - in contrary to gypsum caves - have speleothems. There are no dripstones as in limestone caves, but salt crystals formed by the evaporating of salt rich water at the end of the rain fall. Remaining puddles of water evaporate, the solution becomes supersaturated, and halites grow in multiple forms, but mostlly in cubic form.
There are three famous salt karst areas on the world, in Israel, Spain, and Iran.
At the southern end of the Dead Sea in Israel, a place called Mt Sedom after the biblical town Sedom, is actually a mass of salt. It contains Malham Cave which was the longest salt cave of the world until 2006, and after further explorations it is again since 2019.
The island Queshm in Hormus which belongs to Iran has a huge diapir with the Tri Nahacu Cave or 3n Cave. It was surveyed in 2006 and was the longest salt cave until 2019.
Spain has many salt karst areas, some have caves, others just salt springs and sink holes. A karstiﬁed salt outcrop in Cardona, the Cardona diapir, hosts salt caves whose genesis is related to flooding of mine galleries. The Cova dels Meandres de Sales is 4,300 m long and is the third longest salt cave, other long caves are Sorbas Caves and Estremera Maze Cave. The deepest salt cave in Spain is El Sumidor Cave, VR=210 m.
And finally there is a salt karst in Romania, which is man made, as it developed in the salt bearing spoil tips of a salt mine. It hosts the Pestera 6S de la Mânzálesti, 3,100 m long and the fourth longest salt cave. As it developed in the spoil heaps it is extremely young, only a few decades. And in humid climate the cave development and the solution of the salt is extremely fast. So this cave might be gone in a few decades.