Oceans are the biggest salt reserves on Earth. And the salt deposits on land are mostly deposits created by ancient oceans. The salt deposits we encounter in the Salzkammergut were formed roughly 250 million years ago.
How the salt got into the mountain can be explained by the "bar theory" of Swedish geologist Carl Ochsenius from 1877:
1) A bay was cut off from the rest of the ocean by a shallow (bar) created by the seabed rising or dropping. There was still a connection to the open sea, through which salty sea water flowed continuously.
2) Due to strong evaporation, the concentration of dissolved substances increased. Carbonates, sulphates and salts were thus precipitated.
3) One day all sea water was completely evaporated.
4) During the Mesozoic the salt deposits were covered with rubble and sand from the mainland. Sea water pressing after deposited its sediments over this layer.
5) In the end of the Mesozoic, approx. 60 Mio. years ago the primeval ocean started to shut and the salt deposits were shifted.
6) With the tectonic lifting of the Alps marine sediments were folded, tilted and broken. The very soft and plastic salt deposits were pressed, mixed with the other layers (formation of the “Haselgebirge”) and were brought in the present position.