The ELT will be the largest visible and near-infrared telescope in the world. ESO began its design in early 2006, and aimed to begin construction in 2012. Construction work at the ELT site started in June 2014. As decided by the ESO council on 26 April 2010, a fourth site (Cerro Armazones) is to be home to ELT. Each year about 2,000 requests are made for the use of ESO telescopes, for four to six times more nights than are available. Observations made with these instruments appear in a number of peer-reviewed publications annually; in 2017, more than 1,000 reviewed papers based on ESO data were published. ESO telescopes generate large amounts of data at a high rate, which are stored in a permanent archive facility at ESO headquarters. The archive contains more than 1.5 million images (or spectra) with a total volume of about 65 terabytes (65,000,000,000,000 bytes) of data. La Silla, located in the southern Atacama Desert north of Santiago de Chile at an altitude of , is the home of ESO's original observation site. Like other observatories in the area, La Silla is far from sources of light pollution and has one of the darkest night skies on Earth. In La Silla, ESO operates three telescopes: a 3.6-metre telescope, the New Technology Telescope (NTT) and the 2.2-metre Max-Planck-ESO Telescope. The observatory hosts visitor instruments, attached to a telescope for the duration of an observational run and then removed. La Silla also hosts national telescopes, such as the 1.2-metre Swiss and the 1.5-metre Danish telescopes. About 300 reviewed publications annually are attributable to the work of the observatory. Discoveries made with La Silla telescopes include the HARPS-spectrograph detection of the planets orbiting within the Gliese 581 planetary system, which contains the first known rocky planet in a habitable zone outside the solar system. Several telescopes at La Silla played a role in linking gamma-ray bursts, the most energetic explosions in the universe since the Big Bang, with the explosions of massive stars. The ESO La Silla Observatory also played a role in the study of supernova SN 1987A. The ESO 3.6-metre telescope began operations in 1977. It has been upgraded, including the installation of a new secondary mirror.