The 714,566 hectare site comprises two distinct parts: the dormant volcanic Pinacate Shield of black and red lava flows and desert pavements to the east, and, in the west, the Gran Altar Desert with its ever changing and varied sand dunes that can reach a height of 200 metres. This landscape of dramatic contrast notably features linear, star and dome dunes as well as several arid granite massifs, some as high as 650 metres. The dunes emerge like islands from the sea of sand and harbour distinct and highly diverse plant and wildlife communities, including endemic freshwater fish species and the endemic Sonoran Pronghorn, which is only to be found in northwestern Sonora and in southwestern Arizona (USA). Ten enormous, deep and almost perfectly circular craters, believed to have been formed by a combination of eruptions and collapses, also contribute to the dramatic beauty of the site whose exceptional combination of features are of great scientific interest. The site is also a UNESCO Biosphere Reserve [http://www.unesco.org/new/en/natural-sciences/environment/ecological-sciences/man-and-biosphere-programme/].