One of the Water-Cherenkov detectors of the Auger Observatory being filled. In the background (left) is one of the buildings that houses six of the telescopes used to detect fluorescence radiation produced as the shower particles cross the atmosphere. Signals from the detector are sent to the mast at the fluorescence station and are then transmitted over a microwave link to a computer at the central station. The Andes can be seen in the distance.
The Pierre Auger Observatory Collection