For over 7000 years, 'white gold' has been produced at this site, where over 4km of wooden aqueducts transport high-saline spring water from 200-million-year-old underground salt deposits to a series of terraces where salt is collected after evaporation. Fascinating tours (English available) explain the history and process. From April to October, it also runs one-hour workshops (adult/child €8/5), where you can produce your own salt using traditional methods.
It's 30km west of Vitoria-Gasteiz; you'll need your own transport.