Originally formed by volcanic action that accompanied the uplifting and faulting of the East African shield and the Rift Valley system, Djibouti consists of a series of high, arid tablelands surrounding faults, within which are low plains. Many areas exhibit thick layers of lava flow. There are three principal regions: the coastal plain, less than 200 m (656 ft) above sea level; the mountains, averaging about 1,000 m (3,300 ft) above sea level; and the plateau behind the mountains, rising from 300 to 1,500 m (984–4,921 ft). The highest point, Mt. Moussa Ali, rises to 2,028 m (6,654 ft) on the northern frontier. The saline Lake Assal, at 155 m (509 ft) below sea level, is the lowest point in Africa and the second lowest in the world. In general, the terrain is bare, dry, desolate, and marked by sharp cliffs, deep ravines, burning sands, and thorny shrubs. There is very little groundwater except in an area along the southern border with Somalia, and Djibouti is dependent on saline subterranean aquifers. Earthquakes are common.