About forty kilometres south of Cairo, close to the town of Helwan, lie the ruins of the Sadd-el-Kafara ( = "dam of the Pagans"), an embankment dam of great size built around 2700-2600 BC, discovered over 100 years ago in the old, deep and dry Garawi ravine. The masonry-faced earthen dam originally measured 14 m height and 113 m length along the crest and is considered today the oldest dam of such size known in the world. The primal aim of the dam was to retain the water from rare but violent floods. It could also ensure water to workers...