This is an unbroken mass of sand stretching as much as 400km east to west across the frontiers of Libya and Egypt, lapping at the edges of the Farafra depression to the east. From north to south it spans over 600km from Siwa in the north to the Gilf Kebir plateau (also known as the Great Barrier) in the south. It is mainly made up of parallel, crisply defined dunes orientated more or less north-south. These sharp-edged dunes are known as seif – the Arabic word for sword. Some of the individual dunes are up to 100km long and can be as much as 100m high. While these seif dunes move and are aligned due to the influence of the wind, there are other dunes known as whale-back dunes which are even wider- some up to 2km wide! This extraordinary width maintains their stability and makes limited navigation possible in such a vast, barren but staggeringly beautiful region.