A Silent Civilisation. One of the most mysterious of these four civilisations is no doubt that which developed around the Indus, discovered only in 1920. Considered to be one of the world's most powerful rivers, the Indus has a flow rate twice as high as the Nile, and three times that of the Tigris and the Euphrates. Its geographical course enabled extensive communication to be developed and, at the same time, the rise of a prosperous civilisation, the Harappeans.What was the story of this recently discovered civilisation, and how can we explain its decline?