arid Libyan Desert (Eastern Saharan) basin in northwestern Egypt. Covering about 7,000 square miles (18,100 square km) and containing salt lakes and marshes, it descends to 435 feet (133 m) below sea level. During World War II, because it was impassable to military traffic, the depression formed a natural anchor at the southern end of the British defense lines at El-Alamein (Al-ʿAlamayn; in northwestern Egypt) against the final advance of Field Marshal Rommel's German army in July 1942. In the late 1970s oil deposits were discovered in the southern part of the depression.