The Atacama Desert is a 40,000 square mile plateau in Chile. Its altitude varies, but 7,000 feet is common and mountains on it reach above 22,000 feet.
It is clear and dry with an average of 30 cloudy days per year, and an average annual rainfall of about a millimeter. This leaves many cloudless days/nights suitable for observation, and also means less absorption from water lines, i.e., more substantial atmospheric windows for far infrared and microwave.
Observatories and telescopes include:
(La Silla Observatory is at the edge of the desert.)
Plans and under construction: