The Atacama Desert in northern Chile is generally considered the driest place on Earth outside the polar regions. It averages only about 15 millimeters (0.6 inches) of rain per year, some spots get just 1 to 3 millimeters, and some weather stations there have never recorded rain at all.
Its extreme dryness comes from a two-sided rain shadow: the Andes and the Chilean Coast Range block moisture from both the Atlantic and Pacific, reinforced by the cold Humboldt Current. If polar deserts are included, Antarctica’s McMurdo Dry Valleys are also among the driest places on the planet.
Source: Wikipedia, Atacama Desert