Controlling mercury extraction enabled control of the market, which very quickly became intercontinental in scope because of its decisive role in the extraction of silver from deposits in the New World. A heavy metal, which is liquid at room temperature and has very specific chemical and physical properties, mercury is also a pollutant, which is dangerous for human health. Criterion (ii) : Mercury extraction took place in a very limited number of mines, of which the two largest were Almadén and Idrija. Criterion (iv) : The mining sites of Almadén and Idrija constitute the most important heritage left behind by the intensive extraction of mercury, particularly in the modern and contemporary periods. IntegrityThe mining sites of Almadén and Idrija form a coherent whole with complementary components, satisfactorily illustrating all the technical, cultural and social aspects associated with mercury extraction.