At one time, the salt business along the coast of Puerto de Mazarrón in southern Spain was a thriving enterprise, reaching its peak in the early 20th century. In order to get clean seawater, the manufacturers needed to avoid the beach itself, so they tunneled through a rocky headland to reach deeper, where it would be sediment-free. Those tunnels, now abandoned, are still walkable today.
Back when las salinas (the saltworks) were in use, the water was pumped in and linked to a canal, then channeled on to evaporation pans where the heat of the sun did the job, drying up the H₂O and leaving behind high quality sal de marina. You could call it one of the earliest solar-powered businesses.
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