The extraction of sodium chloride salts of the Sal-Gema mine, between 230 and 314 metres (the uppermost floor of the mine is at 30 metres below sea level), began in the second half of the 20th century and today consists of about 40 kms of underground galleries. These salts, like other precipitation salts and minerals, result from the evaporation of salt water in warm and relatively shallow marine environments, that  were formed during the beginning of the distensional tectonic regime, associated with the fracturing of Pangea and the subsequent opening of the Atlantic Ocean. Thus, during the upper Triassic (237-201 Ma)-lower Jurassic (201-175 Ma), the evaporitic sequences of rock-salt (Halite), anhydrite and gypsum were deposited, with intercalations of clay and volcano-sedimentary material.

Subsequently, the pressure imposed on the salt layers by the carbonate rocks that were deposited afterwards, as well as the tectonic movements, led to a slow movement of the salt towards the surface, forming real salt domes in the middle of the surrounding rocks. These domes are called diapirs and it is thanks to the existence of one of these salt domes below the town of Loulé that this mine was able to develop.