One of the things that makes the Atacama unique, even among deserts, is how long it has been so dry. The Atacama is the oldest desert on Earth and has been semiarid for an estimated 150 million years; the driest part of the desert receives only one-fiftieth of the annual precipitation that Death Valley receives, on average.
This is in part due to the geographic location — and formations — around the Atacama Desert, which has the Andes Mountains on the east and the Chilean Coastal Range on the west, creating a double "rain shadow" where very little precipitation ever falls. Despite this lack of rain, the Atacama is also quite cool. Unlike more stereotypical deserts — like the Saraha — where triple-digit temperatures are the norm, the average temperature ranges between lows in the 30s and highs in the 70s (Fahrenheit), or 0 to the 20s in Celsius.
This might seem like a strange combination of factors, but it has its benefits. Virtually no rain and cool temperatures create a unique ecosystem that has long inspired scientists to explore the life (or lack thereof) in the Atacama — and consider what it might mean for life on other planets too. Unlike other deserts, there are no cacti, insects or even pathogens in some parts of this plain.