The Nile delta is a edge of highly fertile land the size of Northern Ireland situated where the Nile flows out to meet the Mediterranean. It has sustained farming for more than 7,000 years (longer than anywhere on Earth). It is composed of layer upon layer of silt, most of it eroded from the highlands of Ethiopia over tens of thousands of years. Each year, 100 million tonnes of soil erodes from the lands of Wollo and Tigre, and flows north as the Blue Nile floods. The silt raises the Nile delta by about one millimetre each each, fertilizing it and counteracting natural subsidence and erosion by the sea -- that is until the Aswan Dam was completed. As well as replacing the annual flood with predictable and controllable irrigation flows, the dam also trapped 98% of the silt. Denied the fertile silt, Egypt already uses more fertilizer per hectare than any other nation. Its population is expected to double within 30 years.