All salt deposits are of marine origin and were formed about 250 million years ago. Due to the fact that at that time climate in Europe was dry and hot, quite similar to today's coastal Mediterranean climate, sea water was becoming more and more dense. The solutions saturated with salt fell to the bottom and concentration gradually increased.
As a result of tectonic processes a layer of sand was slowly formed on the decks which made the concentration increase and the rock salt crystallized. This process was repeated many times due to the increase of water level in the sea which was a result of geological processes.
Over the centuries, the deposits could be found deeper and deeper and layers were constantly covered with large sediment. At greater depths temperature and pressure were high, which caused tectonic changes: landslides, compression. In this way, deposit of different location and composition were formed.
In Poland, for example, salt deposits were formed 225-210 million years ago in the process of evaporation of the Cechsztynski Sea and they can be found at a depth of 3-6 km below the surface. The oldest traces of salt exploitation from brine springs discovered in Inowrocław come from the seventh-eighth century BC.