What's inside one of those tanks?

The Auger Observatory will have two sites, one in the southern hemisphere and one in the north. This allows scientists to view ultra-high energy cosmic rays over the entire sky. The southern site near the city of Malargüe in Mendoza province, Argentina has been under construction since 2000 and completion is expected in 2007. The northern site is planned for southeast Colorado, near the city of Lamar.

Each one of these plastic tanks is filled with 12 tons of ultra-pure water. When charged particles from a cosmic ray shower zip through the tank, they emit tiny flashes of light which are seen by three very sensitive photomultiplier tubes. These tubes convert light into electrical signals. The source of power is the sun: solar panels are used to charge batteries, which provide all the power the tank needs. Data processing electronics, mounted inside a dome on top of the tank, collects the phototube signals and transmits the processed information via antenna to the main campus. A GPS device provides accurate timing, so that signals from many tanks can be properly compared.