The first stage of the construction of the southern Pierre Auger Observatory in Argentina has finished. This stage consisted of the deployment of a smaller version of the observatory called the Engineering Array. The operation and testing of this array, which included data acquisition of events produced by ultra high energy cosmic rays for several months, have demonstrated the reliability of the Pierre Auger Observatory in order to accomplish the objectives for which it was designed: to answer the most enigmatic questions about the ultra high energy cosmic rays. © 2003 American Institute of Physics