The property encompasses two mercury mining sites. In Almadén mercury has been extracted since Antiquity, while in Idrija it was first found in 1490 A.D. The Almadén site includes buildings relating to its mining history, including Retamar Castle, religious buildings and traditional dwellings. The site in Idrija notably features mercury stores and infrastructure, as well as miners’ living quarters, and a miners’ theatre. The sites bear testimony to the intercontinental trade in mercury which generated important exchanges between Europe and America over the centuries. The two sites represent the two largest mercury mines in the world and were operational until recent times.
Idrija begun the process to inscribe the mercury heritage to the UNESCO's heritage list in 2006. Initially, the nomination was made together with Huancavelica mercury mine in Peru, in relation to the intercontinental route Camino Real. In the second stage, a nomination focused on mercury heritage in relation to silver mining, together with San Luis Potosí mine in Mexico - however, the nomination did not gather sufficient support. The third, successful stage of the nomination focused on mercury mining in relation to technological and industrial processes that influenced the economical and cultural development of the two regions.