The desert is widely known as the driest place on earth, especially around the abandoned mining town of Yungay (where the University of Antofagasta's Desert Research Station is located).
The average rainfall in the Atacama is about 15 mm per year, although in some places 1 to 3 mm has been measured. There are also areas where no precipitation has been recorded. Existing evidence suggests that the Atacama may have remained without significant rainfall from 1570 to 1971.
It is thought that the Atacama may be the oldest desert on Earth and has been hypersaline since at least the middle Miocene, although there have been periods of increased moisture. In some parts of the Atacama, dry conditions have prevailed since the Triassic, 200 million years ago.
The Atacama owes its extreme dryness to a continuous temperature inversion (a reversal of the rate of change of air temperature). Normally, air temperature decreases with increasing altitude, but during an inversion warmer air is held above colder air) caused by the cold Humboldt Ocean Current flowing northward and the presence of a strong Pacific anticyclone.
The driest region of the desert is located between two mountain ranges, the Andes and the Chilean coastal range.