The Atacama Large Millimeter Array observes the universe in the millimetre and submillimetre wavelength ranges, and is the world's largest ground-based astronomy project to date.

It was completed in March 2013 in an international collaboration by Europe (represented by ESO), North America, East Asia and Chile.

[6] Its findings include the discovery of the most distant gamma-ray burst and evidence for a black hole at the centre of the Milky Way.

[13] It was initially planned to set up telescopes in South Africa where several European observatories were located (Boyden Observatory), but tests from 1955 to 1962 demonstrated that a site in the Andes was preferable: When Jürgen Stock enthusiastically reported his observations from Chile, Otto Heckmann decided to leave the South African project on hold.

The new draft was examined in detail, and a council member of CERN (the European Organization for Nuclear Research) highlighted the need for a convention between governments (in addition to organisations).

The final 1962 version was largely adopted from the CERN convention, due to similarities between the organisations and the dual membership of some members.

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.

La Silla, located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft), is the home of ESO's original observation site.

[37] The conventionally designed horseshoe-mount telescope was primarily used for infrared spectroscopy; it now hosts the HARPS spectrograph, used in search of extra-solar planets and for asteroseismology.

This technology (developed by ESO and known as active optics) is now applied to all major telescopes, including the VLT and the future ELT.

The telescope dome is relatively small and ventilated by a system of flaps directing airflow smoothly across the mirror, reducing turbulence and resulting in sharper images.

[40] The 2.2-metre telescope has been in operation at La Silla since early 1984, and is on indefinite loan to ESO from the Max Planck Society (Max-Planck-Gesellschaft zur Förderung der Wissenschaften, or MPG, in German).

[53][54] The main facility at Paranal is the VLT, which consists of four nearly identical 8.2-metre (27 ft) unit telescopes (UTs), each hosting two or three instruments.

[27] The VLT's scientific discoveries include imaging an extrasolar planet,[58] tracking individual stars moving around the supermassive black hole at the centre of the Milky Way[59] and observing the afterglow of the furthest known gamma-ray burst.

[60] At the Paranal inauguration in March 1999, names of celestial objects in the Mapuche language were chosen to replace the technical designations of the four VLT Unit Telescopes (UT1–UT4).

An essay contest was prior arranged for schoolchildren in the region concerning the meaning of these names which attracted many entries dealing with the cultural heritage of ESO's host country.

[61] The four unit telescopes, UT1, UT2, UT3 and UT4, are since known as Antu (sun), Kueyen (moon), Melipal (Southern Cross), and Yepun (Evening Star),[62] with the latter having been originally mistranslated as "Sirius", instead of "Venus".

[63] Visible and Infrared Survey Telescope for Astronomy (VISTA) is housed on the peak adjacent to the one hosting the VLT, sharing observational conditions.

The VST (which became operational in 2011) is the result of a joint venture between ESO and the Astronomical Observatory of Capodimonte (Naples), a research centre at the Italian National Institute for Astrophysics INAF.

Astronomers use this light to study the chemical and physical conditions in these molecular clouds, the dense regions of gas and cosmic dust where new stars are being born.

Seen in visible light, these regions of the universe are often dark and obscure due to dust; however, they shine brightly in the millimetre and submillimetre portions of the electromagnetic spectrum.

[72][73] ESO hosts the Atacama Pathfinder Experiment, APEX, and operates it on behalf of the Max Planck Institute for Radio Astronomy (MPIfR).

[74] The antennas can be arranged across the desert plateau over distances from 150 metres to 16 kilometres (9.9 mi), which will give ALMA a variable "zoom".

The array will be able to probe the universe at millimetre and submillimeter wavelengths with unprecedented sensitivity and resolution, with vision up to ten times sharper than the Hubble Space Telescope.

[75] ALMA is a collaboration between East Asia (Japan and Taiwan), Europe (ESO), North America (US and Canada) and Chile.