An array of over 200 water-Cherenkov detectors covering 12 km2 was operated for over 20 years from 1967 at Haverah Park in England. Although designed before the GZK prediction, the data have contributed in a major way to our understanding of cosmic rays at the highest energies. For the Auger Observatory, the demonstration that water could be kept bacteria-free in a sealed container for over 25 years was of major importance as were early studies of inclined showers and of the time structure of the shower front.
A tank was opened at the 'end of project' party on 31 July 1987. The water shown in the photo had been in the tank for 25 years but was quite drinkable!