The Sahara's topographical features include shallow basins, large oasis depressions, serirs or regs (gravel-covered plains), plateaus, mountains, sand sheets, dunes and sand seas (ergs). The highest part of the desert is at the summit of Mount Koussi, which is 11,204 feet (3,415 m) high. However, the lowest point of the Sahara is 436 feet (133 m) below sea level: in the Qattera Depression in Egypt.
Over 25 percent of the Sahara's surface is covered by sand sheets and dunes. The most common types of dunes include tied dunes, blowout dunes, barchan and transverse dunes, longitudinal seirfs, and complex sand seas. Within the Sahara are several pyramidal dunes that reach over 500 feet in height while the draa, a mountainous sand ridge, reaches over 1,000 feet. Researchers have for many years tried to figure out how these dunes were formed, but the case remains unsolved.
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