SUMMARY: The 100th detector for the Pierre Auger Observatory was recently completed, making the array the world's largest cosmic ray detector. It consists of surface detectors spread out over 181 square kilometers of land in Argentina. Once it's working, the detector should be able to capture some of the most energetic cosmic ray particles - they only strike a 2.5 square kilometer area once a year. The mystery with these high-energy particles is that astronomers have no idea what in the Universe could create them. The long term plans for the observatory is to eventually have 1,600 detectors by 2005.

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