At more than 6.700 kilometers in length, the river Nile, including its main tributaries the Blue Nile and the White Nile, is the longest river in the world. For a country such as Egypt, located in one of the driest places on earth, the river's waters have always been of vital importance. Apart from water itself, the river provided the basis for agriculture in the region by leaving behind a fertile layer of mud across the banks of the river after each annual inundation. As a result, Egypt is often referred to as 'The gift of the Nile'. The ancient Egyptians believed the waters of the Nile flowed forth from Nun, the primeval waters beneath the earth, and surged upwards through the cataract at Elephantine. Several Nilometers were built along the river to measure the coming inundation, personified as the god Hapi and thought to be work of the god Khnum, who was revered on the island.