The Qattara Depression contains the second lowest point in Africa at an altitude of 133 metres (436 ft) below sea level, the lowest point being Lake Assal in Djibouti. The depression covers about 19,605 square kilometres (7,570 sq mi), a size comparable to Lake Ontario or twice as large as Lebanon. Due to its size and proximity to the Mediterranean Sea shore, it has been studied for its potential to generate hydroelectricity.
The depression has the shape of a teardrop, with its point facing east and the broad deep area facing the south west. The northern side of the depression is characterised by steep escarpments up to 280 meters high, marking the edge of the adjacent El Diffa plateau. To the south the depression slopes gently up to the Great Sand Sea.
Within the Depression are salt marshes, under the northwestern and northern escarpment edges, and extensive dry lake beds that flood occasionally. The marshes occupy approximately 300 square kilometres (120 sq mi), although wind-blown sands are encroaching in some areas. About a quarter of the region is occupied by dry lakes composed of hard crust and sticky mud, and occasionally filled with water.
The depression was initiated by either wind or fluvial erosion in the late Neogene, but during the Quaternary the dominant mechanism has been a combination of salt weathering and wind erosion working together. First, the salts break up the depression floor, then the wind blows away the resulting sands. This process is less effective in the eastern part of the depression, due to lower salinity groundwater.
Groves of Acacia raddiana, growing in shallow sandy depressions, and Phragmites swamps represent the only permanent vegetation. The acacia groves vary widely in biodiversity and rely on runoff from rainfall and groundwater to survive. The Moghra Oasis in the northeast of the Depression has a 4 km2 brackish lake and a Phragmites swamp.