Atacama Desert : 1 millimetre rainfall per year

The Atacama Desert is a plateau in South America, covering a 600-mile (1,000 km) strip of land on the Pacific coast of South America, west of the Andes mountains. The Atacama occupies 40,600 square miles (105,000 km²) in northern Chile, composed mostly of salt basins, sand, and felsic lava flows towards the Andes.

Atacama Desert is commonly known as the driest place in the world. These incredibly dry mountains see an average of less than .004 inches/.01cm of rain per year. Many times this area will go without rainfall at all for years. Some places in the Atacama Desert have not had rainfall for over 400 years. 

Unlike more familiar deserts, like the Sahara desert in Africa and the Mojave in California, the Atacama is actually a pretty cold place, with average daily temperatures ranging between 0°C and 25°C.

Because of its high altitude, nearly non-existent cloud cover, dry air, and lack of light pollution and radio interference from the very widely spaced cities, the desert is one of the best places in the world to conduct astronomical observations. The European Southern Observatory operates two major observatories in the Atacama: The La Silla Observatory and the Paranal Observatory, which includes the Very Large Telescope. A new radio astronomy telescope, called ALMA, is being built by Europe, Japan, the United States, Canada and Chile in the Llano de Chajnantor Observatory. A number of radio astronomy projects, such as the CBI, the ASTE and the ACT, among others, have been operating in the Chajnantor area since 1999.

      Paranal Observatory

For the most part, Atacama is a pretty lonely place. Humans have lived in the Atacama for many thousands of years, based on the cultural relics and artifacts that archaeologists have found. The South American Indians who have set up housekeeping in the desert over the millennia have left relics from their culture and even themselves. Because the Atacama is so bone-dry the bodies of the buried indians have dried perfectly preserved turning them into mummies. Some of the oldest mummies found anywhere on earth have come from the Atacama Desert and have been dated to be 9,000 years old.

The desert has rich deposits of copper and other minerals, and the world's largest natural supply of sodium nitrate, which was mined on a large scale until the early 1940s. The Atacama border dispute over these resources between Chile and Bolivia began in the 19th century.

A reason that the Atacama doesn't get enough rainfall is because of a phenomenon called rainshadow. The warm, moist tropical air that blows on the tradewinds from the east, which douse the South American rainforest, get hung-up on the east side of the Andes. The mountains are so high in altitude that the air cools, condenses and rains (or snows) on the mountains. As the air descends the other side of the mountain range it warms, holding in its moisture preventing rain from falling on to the ground below.

This is one of the reasons why the Amazon basin and river are the largest anywhere in the world. The mountains that cause the Amazon to be the largest river from collecting all the rainfall are also responsible for preventing the Atacama from ever receiving any rainfall. The driest and one of the wettest places in the world are right next to each other.