The Salar de Uyuni varies in elevation by less than one metre across 10,582 square kilometres, which is why space agencies use its crust to calibrate satellite altimeters. The plain is the drained floor of Lake Tauca, a paleolake that covered much of the Bolivian altiplano roughly 14,000 years ago. When the water evaporated it left a salt crust up to ten metres thick, resting on brine unusually rich in lithium. Beneath the white hexagons lie some of the largest identified lithium reserves on earth. San Pedro de Atacama to Uyuni 4 Day Tour tours follow a corridor that llama caravans and mule trains used centuries before tourism reached the border. From the Chilean side the route climbs to the Hito Cajón post at roughly 4,480 metres, then enters the Eduardo Avaroa Andean Fauna National Reserve, gazetted in 1973 and covering 7,147 square kilometres. Laguna Verde lies at the foot of Licancabur, a 5,916-metre stratovolcano whose summit still holds Inca ritual platforms. Northward, the Sol de Mañana geyser field vents sulphurous steam at about 4,850 metres. Laguna Colorada, coloured by red algae and borax islands, shelters one of the densest known populations of James's flamingos, a bird believed extinct until 1957. Vehicles running the San Pedro de Atacama to Uyuni 4 Day Tour tour route now pay reserve fees that help fund ranger patrols across that terrain. The salar itself supports two economies that sit uneasily together. Salt is still cut by hand at Colchani, raked into conical mounds to drain, then iodised and bagged for domestic sale. A short distance away, the rail yard outside Uyuni holds British-built locomotives abandoned when the mineral trade contracted in the 1940s, corroding in wind that carries salt. Together, Salar de Uyuni crossings and wider Bolivian altiplano landmarks have given Daniel Campos Province one of the department's few dependable sources of income outside mining. Incahuasi Island, a fossilised coral outcrop set some 80 kilometres inside the flats, carries Trichocereus pasacana cacti that grow roughly a centimetre a year; the tallest exceed ten metres and were seedlings before Bolivia existed as a republic. Stone shelters on its slope were raised by Quechua herders who drove salt caravans toward the valleys and returned with maize. Itineraries that include the island under San Pedro de Atacama to Uyuni 4 Day Tour tickets treat it as the crossing's geological full stop, evidence that this plain was once an inland sea.