HELYCON, the HEllenic LYceum Cosmic Observatories Network, is a network of detector stations distributed over greater Patra, and the Chios and Cyprus Islands. HELYCON aims to observe Extensive Air Showers of very energetic cosmic rays. It is also used for the calibration of KM3NeT, the Mediterranean neutrino telescope. We report on the design, construction and performance of this prototype detector array.