is extracted (by solution mining) out of ancient seabeds, for example, the Zechstein seabed in northwest Europe. Some deposits result from high content of magnesium chloride in the primordial ocean. Some magnesium chloride is made from evaporation of seawater.

In the Dow process, magnesium chloride is regenerated from magnesium hydroxide using hydrochloric acid:
:It can also be prepared from magnesium carbonate