They built the iconic pyramids and the Great Sphinx of Giza, which still stand as a monument to their skills and tenacity to this day.  But the fall of the great Egyptian Old Kingdom may have been helped along by a common problem which remains with us now - drought.  Researchers from the University of St andrews have confirmed that a severe period of drought around 4,200 years ago may have contributed to the demise of the civilisation.  Using seismic investigations with sound waves, along with carbon dating of a 100-metre section of sediment from the bed of Lake Tana in Ethiopia, the team were able to look back many thousands of years. They were able to see how water levels in the lake had varied over the past 17,000 years, with the sediment signalling lush periods but also times of drought.  Lake Tana - the source of the Blue Nile river - flows to the White Nile at Khartoum and eventually to the Nile Del