In 1968 hot brines from which gypsum had precipitated were discovered in the Red Sea, in a divergent plate boundary setting with hydrothermal activity¹. We now report the discovery of cold brines in an entirely different geodynamic situation, a convergent plate boundary in the eastern Mediterranean, where the heatflow is very low² and no hydrothermal activity occurs. Decimetric pure gypsum crystals precipitate from the cold brines, which occupy the bottom of a rimmed anoxic basin near the southern edge of the Mediterranean Ridge, close to the Sirte Abyssal Plain. Nucleation of the crystals does not occur at the surface, but from the deep brines, because the salinity of the water is normal down to the depth of −3,000 m. We propose the name ‘Bacino Bannock’ for this basin, after the Consiglio Nazionale delle Ricerche R/V Bannock from which the bottom-water brines were surveyed and sampled.