One of the 1,600 Detectors at the Pierre Auger Observatory (image) CNRS Share Print E-Mail Caption The detectors are tanks filled with 12 tonnes of pure water, which are used to detect particles from air showers, cascades of secondary particles produced when cosmic rays enter Earth's atmosphere. As they travel through the tanks of water, the secondary particles produce a flash of light caused by the Cherenkov effect. The Pierre Auger Observatory in Argentina is the world's largest cosmic ray detector. It is named after the French physicist who was the first to observe air showers, in 1938. Credit Céline ANAYA-GAUTIER/CNRS Photothèque Usage Restrictions The image may only be used with appropriate caption or credit. Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.