In internet terms this is a relatively ancient mystery, being first identified by a German, KenGrok in 2006. Much has already been written about the 1:500 replica landscape constructed in the deserts of the Ningxia Hui autonomous region of China, but the consensus is that this remarkable facility is in fact a scale facsimile (complete with snow-peaked mountains, valleys, tributaries and lakes) of the Aksai Chin contested region on the Indian-Chinese border 2400km away. It has been suggested that this 700 x 900 metre facility is used for tank training but the size and scale of the replica suggests a use in reconnaissance or visualization training of some kind. This image from the Sidney Morning Herald (obtained from a Chinese web forum) seems to show technicians at the Huangyangtan site or a similar terrain fabricated elsewhere. Whatever its purpose, I was reminded of the one paragraph short story by Jorge Luis Borges, ‘On Exactitude in Science’, which I've copied out in full here:

… In that Empire, the art of cartography attained such Perfection that the map of a single Province occupied the entirety of a City, and the map of the Empire the entirety of the Province. In time those Unconscionable Maps no longer satisfied, and the Cartographer Guild struck a Map of the Empire whose size was that of the Empire, and which coincided point to point with it. The following Generations, who were not so fond of the Study of Cartography as their Forebears had been, saw that vast Map was Useless, and not without some Pitilessness was it, that they delivered it up the Inclemencies of Sun and Winters. In the Deserts of the West, still today, there are Tattered Ruins of the Map, inhabited by Animals and Beggars; in all the Land there is no other Relic of the Discipline of Geography.

Suárez Miranda, Viajes de varones Prudentes

Libro IV, Cap. XLV, Lérida, 1658