I haven't ever seen these in real life, but the following image is of the access to an underground grain storage "facility". These were used in much of the Mediterranean from the 1600s to the 1900s, and some restored examples are visitable in Puglia: this pic is of one in Cerignola, FG.
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And here is a drawing of the construction of these underground grain silos.
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Or, for one of the largest "saline" (salt collection fields) in Europe there is Margherita di Savoia - not only the salt pans, but lots of thermal baths with sodium bromide and iodine in the waters.
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