Which is the driest desert on Earth?

The Atacama, in northern Chile. Some weather stations there have recorded no rainfall in the entire period they have been running, and there is riverbed evidence of stretches of centuries with essentially none. It sits in a double rain shadow - the Andes block moisture from the Amazon, and a cold current offshore stops the Pacific from delivering any either.

The result is a landscape that behaves like another planet, which is precisely what it gets used for. NASA tests Mars instruments there, and early soil experiments found sites so sterile that standard methods could detect no life at all. It is also where some of the best-preserved mummies in the world are found: the Chinchorro people were deliberately mummifying their dead two thousand years before the Egyptians, and the ground did most of the work.

One caveat on the superlative. By pure lack of precipitation the McMurdo Dry Valleys in Antarctica are comparable or drier - a cold desert where it has not rained for perhaps two million years. The Atacama usually gets the title because people expect a desert to be hot.