File:Hidden in the Chilean Andes (potw2335a).jpg

English: The flat panorama of the Chajnantor plateau dominates this Picture of the Week. Situated at an altitude of around 5000 metres in the Chilean Andes, this plateau is one of the driest places on Earth — challenging for humans, but perfect for astronomical observations.This breathtaking landscape is home to the Atacama Pathfinder Experiment (APEX), the white structure down in the centre of the image. Inaugurated in 2005, APEX is a 12-metre diameter radio telescope that studies the cold, dusty and distant Universe, and is operated by ESO on behalf of the Max Planck Institute for Radio Astronomy (MPIfR).Now look ahead, towards the distant brown mountains, and you might be able to make out a bunch of white dots. Those are in fact the 66 high-precision antennas of ALMA, the Atacama Large Millimeter/submillimeter Array, operated by ESO together with its international partners. Like APEX, ALMA’s antennas observe the coldest objects in the Universe, such as vast clouds of gas and dust only a few tens of degrees above absolute zero. But these antennas work together as a single, huge “virtual” antenna, capable of discerning small details.But why build these advanced instruments in a remote place hidden in the Chilean desert? The signals emitted from the coldest sources of the Universe are heavily absorbed by water vapour in the Earth’s atmosphere. That's why APEX and ALMA are located in such a high and dry place: to minimise the amount of water vapour above them and therefore better detect the signals