Mirror reflection of a church on the water of the salt pans at Messolonghi, Greece. Salt, sodium chloride, has played a crucial role in the history of civilization and in ancient Greece it was used as a commodity for trading slaves. It is said that the expression 'not worth his salt' came from this practice. The Messolonghi Aetoliko lagoon system is considered to be one of the most important Mediterranean salt lagoons and is certainly the largest in Greece, producing 60 percent of the country's salt. A basic advantage of natural sea salt, as opposed to its mineral form, is its crystalline structure, which contains trace elements of marine water, such as potassium, calcium, magnesium, etc. American naturalist Henry David Thoreau spoke of such a lake, at Walden, as 'a field of water, intermediate in its nature between land and sky'.