On hot and calm days, a very thin layer of salt crystals forms on the surface of the salt pan. This is carefully collected by workers who scrape off only the top layer. Once dry, it becomes salt flower – "fleur de sel", "flor de sal". The Salines of Rio Maior, one of the greatest natural resources of the Ribatejo region, have eight centuries of history. They are very deep and extensive rocksalt mines, crossed by an underground stream that feeds a well. The water extracted from this well is seven times saltier than that of the oceans.