Astrophysicists from around the world gathered near the town of Malargue in Mendoza province of Argentina in the second week of March. Their aim was to break new ground on a US $53-million facility, which they hope will solve one of the great mysteries of the sky: where do cosmic rays come from and what makes them so energetic? Called the Pierre-Auger project, it will be the world's first large-scale cosmic ray detector, eventually comprising 1,600 water tanks with a capacity of 11,000 litres spread over an area of 3,000 sq km. Showers of particles created when extremely high-energy cosmic rays slam the Earth's atmosphere will pass through these tanks. This will create trails of what is known as Cherenkov light, which will be picked up by detectors for analysis ( Science , Vol 283, No 5410).