Because of its high altitude, nearly nonexistent cloud cover, dry air, and lack of light pollution and radio interference from widely populated cities and towns, this desert is one of the best places in the world to conduct astronomical observations. A radio astronomy telescope, called the Atacama Large Millimeter Array, built by European countries, Japan, the United States, Canada, and Chile in the Llano de Chajnantor Observatory officially opened on 3 October 2011. 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. On 26 April 2010, the ESO council decided to build a fourth site, Cerro Armazones, to be home to the Extremely Large Telescope. Construction work at the ELT site started in June 2014.