The Sahara (, ) is a desert spanning across North Africa. With an area of , it is the largest hot desert in the world and the third-largest desert overall, smaller only than the deserts of Antarctica and the northern Arctic.
The name "Sahara" is derived from , a broken plural form of (صَحْرَاء ), meaning "desert". "Sahara." Online Etymology Dictionary. Douglas Harper, Historian. Retrieved 25 June 2007.
The desert covers much of North Africa, excluding the fertile region on the Mediterranean Sea coast, the Atlas Mountains of the Maghreb, and the Nile in Egypt and the Sudan.
It stretches from the Red Sea in the east and the Mediterranean in the north to the Atlantic Ocean in the west, where the landscape gradually changes from desert to coastal plains. To the south it is bounded by the Sahel, a belt of semi-arid tropical savanna around the Niger River valley and the Sudan region of sub-Saharan Africa. The Sahara can be divided into several regions, including the western Sahara, the central Ahaggar Mountains, the Tibesti Mountains, the Aïr Mountains, the Ténéré desert, and the Libyan Desert.
For several hundred thousand years, the Sahara has alternated between desert and savanna grassland in a 20,000-year cycle caused by the precession of Earth's axis (about 26,000 years) as it rotates around the Sun, which changes the location of the North African monsoon.
The Sahara is mainly rocky hamada (stone plateaus); ergs (sand seas – large areas covered with sand ) form only a minor part, contrary to common misconception, but many of the are over high.Strahler, Arthur N. and Strahler, Alan H. (1987) Modern Physical Geography Third Edition. New York: John Wiley & Sons. . p. 347 Wind or rare rainfall shape the desert features: sand dunes, dune fields, sand seas, stone plateaus, gravel plains ( reg), wadi ( wadi), ( oued), and salt flats ( shatt or chott). Unusual landforms include the Richat Structure in Mauritania.
Several deeply dissected mountains, many volcanic, rise from the desert, including the Aïr Mountains, Ahaggar Mountains, Saharan Atlas, Tibesti Mountains, Adrar des Iforas, and the Red Sea Hills. The highest peak in the Sahara is Emi Koussi, a shield volcano in the Tibesti range of northern Chad.
The central Sahara is hyperarid, with sparse vegetation. The northern and southern reaches of the desert, along with the highlands, have areas of sparse grassland and desert shrub, with trees and taller shrubs in , where moisture collects. In the central, hyperarid region, there are many subdivisions of the great desert: Tanezrouft, the Ténéré, the Libyan Desert, the Eastern Desert, the Nubian Desert and others. These extremely arid areas often receive no rain for years.
To the north, the Sahara skirts the Mediterranean Sea in Egypt and portions of Libya, but in Cyrenaica and the Maghreb, the Sahara borders the Mediterranean forest, woodland, and scrub eco-regions of northern Africa, all of which have a Mediterranean climate characterized by hot summers and cool and rainy winters. According to the botanical criteria of Frank WhiteWickens, Gerald E. (1998) Ecophysiology of Economic Plants in Arid and Semi-Arid Lands. Springer, Berlin. and geographer Robert Capot-Rey, the northern limit of the Sahara corresponds to the northern limit of date palm cultivation and the southern limit of the range of esparto, a Poaceae typical of the Mediterranean climate portion of the Maghreb and Iberia. The northern limit also corresponds to the isohyet of annual precipitation.
To the south, the Sahara is bounded by the Sahel, a belt of dry tropical savanna with a summer rainy season that extends across Africa from east to west. The southern limit of the Sahara is indicated botanically by the southern limit of Cornulaca monacantha (a drought-tolerant member of the Chenopodiaceae), or northern limit of Cenchrus biflorus, a Poaceae typical of the Sahel. According to climatic criteria, the southern limit of the Sahara corresponds to the isohyet of annual precipitation (this is a long-term average, since precipitation varies annually).
Important cities located in the Sahara include Nouakchott, the capital of Mauritania; Tamanrasset, Ouargla, Béchar, Hassi Messaoud, Ghardaïa, and El Oued in Algeria; Timbuktu in Mali; Agadez in Niger; Ghat in Libya; and Faya-Largeau in Chad.
The permanent absence of clouds allows unhindered light and thermal radiation. The stability of the atmosphere above the desert prevents any convective overturning, thus making rainfall virtually non-existent. As a consequence, the weather tends to be sunny, dry and stable with a minimal chance of rainfall. Subsiding, diverging, dry air masses associated with subtropical high-pressure systems are extremely unfavorable for the development of convectional showers. The subtropical ridge is the predominant factor that explains the hot desert climate (Köppen climate classification BWh) of this vast region. The descending airflow is the strongest and the most effective over the eastern part of the Great Desert, in the Libyan Desert: this is the sunniest, driest and the most nearly "rainless" place on the planet, rivaling the Atacama Desert, lying in Chile and Peru.
The rainfall inhibition and the dissipation of cloud cover are most accentuated over the eastern section of the Sahara rather than the western. The prevailing air mass lying above the Sahara is the continental tropical (cT) air mass, which is hot and dry. Hot, dry air masses primarily form over the North-African desert from the heating of the vast continental land area, and it affects the whole desert during most of the year. Because of this heat wave process, a thermal low is usually noticed near the surface, and is the strongest and the most developed during the summertime. The Sahara High represents the eastern continental extension of the Azores High, centered over the North Atlantic Ocean. The subsidence of the Sahara High nearly reaches the ground during the coolest part of the year, while it is confined to the upper troposphere during the hottest periods.
The effects of local surface low pressure are extremely limited because upper-level subsidence still continues to block any form of air ascent. Also, to be protected against rain-bearing weather systems by the atmospheric circulation itself, the desert is made even drier by its geographical configuration and location. Indeed, the extreme aridity of the Sahara is not only explained by the subtropical high pressure: the Atlas Mountains of Algeria, Morocco and Tunisia also help to enhance the aridity of the northern part of the desert. These major mountain ranges act as a barrier, causing a strong rain shadow effect on the leeward side by dropping much of the humidity brought by atmospheric disturbances along the polar front which affects the surrounding Mediterranean climates.
The primary source of rain in the Sahara is the Intertropical Convergence Zone, a continuous belt of low-pressure systems near the equator which bring the brief, short and irregular rainy season to the Sahel and southern Sahara. Rainfall in this giant desert has to overcome the physical and atmospheric barriers that normally prevent the production of precipitation. The harsh climate of the Sahara is characterized by: extremely low, unreliable, highly erratic rainfall; extremely high sunshine duration values; high temperatures year-round; negligible rates of relative humidity; a significant diurnal temperature variation; and extremely high levels of potential evaporation which are the highest recorded worldwide.
The high position of the Sun, the extremely low relative humidity, and the lack of vegetation and rainfall make the Great Desert the hottest large region in the world, and the hottest place on Earth during summer in some spots. The average high temperature exceeds during the hottest month nearly everywhere in the desert except at very high altitudes. The world's highest mean monthly maximum temperature was in a remote desert town in the Algerian Desert called Bou Bernous, at an elevation of above sea level; it is rivaled only by Death Valley, California.
Other hot spots in Algeria such as Adrar, Timimoun, In Salah, Ouallene, Aoulef, Reggane with an elevation between above sea level get slightly lower summer average highs, around during the hottest months of the year. Salah, well known in Algeria for its extreme heat, has average high temperatures of , , and in June, July, August and September respectively. There are even hotter spots in the Sahara, but they are located in extremely remote areas, especially in the Azalai, lying in northern Mali. The major part of the desert experiences around three to five months when the average high exceeds ; while in the southern central part of the desert, there are up to six or seven months when the average high temperature exceeds . Some examples of this are Bilma, Niger and Faya-Largeau, Chad. The annual average daily temperature exceeds everywhere and can approach in the hottest regions year-round. However, most of the desert has a value in excess of .
Sand and ground temperatures are even more extreme. During daytime, the sand temperature is extremely high: it can easily reach or more. A sand temperature of has been recorded in Port Sudan. Ground temperatures of have been recorded in the Adrar Plateau and a value of has been measured in Borkou, northern Chad.
Due to lack of cloud cover and very low humidity, the desert usually has high diurnal temperature variations between days and nights. However, it is a myth that the nights are especially cold after extremely hot days in the Sahara. On average, nighttime temperatures tend to be cooler than in the daytime. The smallest variations are found along the coastal regions due to high humidity and are often even lower than a difference, while the largest variations are found in inland desert areas where the humidity is the lowest, mainly in the southern Sahara. Still, it is true that winter nights can be cold, as it can drop to the freezing point and even below, especially in high-elevation areas. The frequency of subfreezing winter nights in the Sahara is strongly influenced by the North Atlantic Oscillation (NAO), with warmer winter temperatures during negative NAO events and cooler winters with more frosts when the NAO is positive. This is because the weaker clockwise flow around the eastern side of the subtropical anticyclone during negative NAO winters, although too dry to produce more than negligible precipitation, does reduce the flow of dry, cold air from higher latitudes of Eurasia into the Sahara significantly.
The vast central hyper-arid core of the desert is virtually never affected by northerly or southerly atmospheric disturbances and permanently remains under the influence of the strongest anticyclonic weather regime, and the annual average rainfall can drop to less than . In fact, most of the Sahara receives less than . Of the of desert land in the Sahara, an area of about (about 31% of the total area) receives an annual average rainfall amount of or less, while some (about 17% of the total area) receives an average of or less.
The annual average rainfall is virtually zero over a wide area of some in the eastern Sahara comprising deserts of: Libya, Egypt and Sudan (Tazirbu, Kufra, Dakhla Oasis, Kharga, Farafra, Siwa Oasis, Asyut, Sohag, Luxor, Aswan, Abu Simbel, Wadi Halfa) where the long-term mean approximates per year. Rainfall is very unreliable and erratic in the Sahara as it may vary considerably year by year. In full contrast to the negligible annual rainfall amounts, the annual rates of potential evaporation are extraordinarily high, roughly ranging from per year to more than per year in the whole desert. Nowhere else on Earth has air been found as dry and evaporative as in the Sahara region. However, at least two instances of snowfall have been recorded in Sahara, in February 1979 and December 2016, both in the town of Ain Sefra.
The climate of the Sahara has undergone enormous variations between wet and dry over the last few hundred thousand years, believed to be caused by long-term changes in the North African climate cycle that alters the path of the North African Monsoon – usually southward. The cycle is caused by a 41,000-year cycle in which the tilt of the earth changes between 22° and 24.5°. At present, it is in a dry period, but it is expected that the Sahara will become green again in 15,000 years. When the North African monsoon is at its strongest, annual precipitation and subsequent vegetation in the Sahara region increase, resulting in conditions commonly referred to as the "green Sahara". For a relatively weak North African monsoon, the opposite is true, with decreased annual precipitation and less vegetation resulting in a phase of the Sahara climate cycle known as the "desert Sahara".
The idea that changes in insolation (solar heating) caused by long-term changes in Earth's orbit are a controlling factor for the long-term variations in the strength of monsoon patterns across the globe was first suggested by Rudolf Spitaler in the late nineteenth century,
During the last glacial period, the Sahara was much larger than it is today, extending south beyond its current boundaries.Ehret, Christopher, The Civilizations of Africa, University Press of Virginia, 2002. The end of the glacial period brought more rain to the Sahara, from about 8000 BCE to 6000 BCE, perhaps because of low pressure areas over the collapsing to the north. Fezzan Project — Palaeoclimate and environment. Retrieved 15 March 2006. Once the ice sheets were gone, the northern Sahara dried out. In the southern Sahara, the drying trend was initially counteracted by the monsoon, which brought rain further north than it does today. By around 4200 BCE, however, the monsoon retreated south to approximately where it is today, Sahara's Abrupt Desertification Started by Changes in Earth's Orbit , Accelerated by Atmospheric and Vegetation Feedbacks. leading to the gradual desertification of the Sahara. The Sahara is now as dry as it was about 13,000 years ago.
Lake Chad is the remnant of a former inland sea, paleolake Mega-Chad, which existed during the African humid period. At its largest extent, sometime before 5000 BCE, Lake Mega-Chad was the largest of four Saharan paleolakes, and is estimated to have covered an area of 350,000 km2.
The Sahara pump theory describes this cycle. During periods of a wet or "Green Sahara", the Sahara becomes a savanna grassland and various flora and fauna become more common. Following inter-pluvial arid periods, the Sahara area then reverts to desert conditions and the flora and fauna are forced to retreat northwards to the Atlas Mountains, southwards into West Africa, or eastwards into the Nile Valley. This separates of some of the species in areas with different , forcing them to adaptation, possibly giving rise to allopatric speciation.
It is also proposed that humans accelerated the drying-out period from 6000 to 2500 BCE by pastoralists overgrazing available grassland.
During the Last Glacial Maximum (LGM) the Sahara was more extensive than it is now with the extent of the tropical forests being greatly reduced, and the lower temperatures reduced the strength of the Hadley Cell. This is a climate cell which causes rising tropical air of the Inter-Tropical Convergence Zone (ITCZ) to bring rain to the tropics, while dry descending air, at about 20 degrees north, flows back to the equator and brings desert conditions to this region. It is associated with high rates of wind-blown mineral dust, and these dust levels are found as expected in marine cores from the north tropical Atlantic. But around 12,500 BCE the amount of dust in the cores in the Bølling/Allerød phase suddenly plummets and shows a period of much wetter conditions in the Sahara, indicating a Dansgaard-Oeschger (DO) event (a sudden warming followed by a slower cooling of the climate). The moister Saharan conditions had begun about 12,500 BCE, with the extension of the ITCZ northward in the northern hemisphere summer, bringing moist wet conditions and a savanna climate to the Sahara, which (apart from a short dry spell associated with the Younger Dryas) peaked during the Holocene thermal maximum climatic phase at 4000 BCE when mid-latitude temperatures seem to have been between 2 and 3 degrees warmer than in the recent past. Analysis of Nile River deposited sediments in the delta also shows this period had a higher proportion of sediments coming from the Blue Nile, suggesting higher rainfall also in the Ethiopian Highlands. This was caused principally by a stronger circulation throughout the sub-tropical regions, affecting India, Arabia and the Sahara. Lake Victoria only recently became the source of the White Nile and dried out almost completely around 15 kya.
The sudden subsequent movement of the ITCZ southwards with a Heinrich event (a sudden cooling followed by a slower warming), linked to changes with the El Niño-Southern Oscillation cycle, led to a rapid drying out of the Saharan and Arabian regions, which quickly became desert. This is linked to a marked decline in the scale of the Nile floods between 2700 and 2100 BCE.Burroughs, William J. (2007) "Climate Change in Prehistory: the end of the reign of chaos" (Cambridge University Press)
The Saharan flora comprises around 2800 species of . Approximately a quarter of these are endemism. About half of these species are common to the flora of the Arabian deserts.
The central Sahara is estimated to include five hundred species of plants, which is extremely low considering the huge extent of the area. Plants such as acacia trees, palms, succulents, spiny shrubs, and grasses have adapted to the arid conditions, by growing lower to avoid water loss by strong winds, by storing water in their thick stems to use it in dry periods, by having long roots that travel horizontally to reach the maximum area of water and to find any surface moisture, and by having small thick leaves or needles to prevent water loss by evapotranspiration. Plant leaves may dry out totally and then recover.
Several species of fox live in the Sahara including: the fennec fox, pale fox and Rüppell's fox. The addax, a large white antelope, can go nearly a year in the desert without drinking. The dorcas gazelle is a north African gazelle that can also go for a long time without water. Other notable gazelles include the rhim gazelle and dama gazelle.
The Saharan cheetah (northwest African cheetah) lives in Algeria, Togo, Niger, Mali, Benin, and Burkina Faso. There remain fewer than 250 mature cheetahs, which are very cautious, fleeing any human presence. The cheetah avoids the sun from April to October, seeking the shelter of shrubs such as balanites and acacias. They are unusually pale. The other cheetah subspecies (northeast African cheetah) lives in Chad, Sudan and the eastern region of Niger. However, it is currently extinct in the wild in Egypt and Libya. There are approximately 2000 mature individuals left in the wild.
Other animals include the , hyrax, sand vipers, and small populations of African wild dog, in perhaps only 14 countries and red-necked ostrich. Other animals exist in the Sahara (birds in particular) such as African silverbill and black-faced firefinch, among others. There are also small in Mauritania and the Ennedi Plateau of Chad." Desert-Adapted Crocs Found in Africa ", National Geographic News, 18 June 2002
The deathstalker scorpion can be long. Its venom contains large amounts of agitoxin and scyllatoxin and is very dangerous; however, a sting from this scorpion rarely kills a healthy adult. The Saharan silver ant is unique in that due to the extreme high temperatures of their habitat, and the threat of predators, the ants are active outside their nest for only about ten minutes per day.
Dromedary camels and are the domesticated animals most commonly found in the Sahara. Because of its qualities of endurance and speed, the dromedary is the favourite animal used by .
Human activities are more likely to affect the habitat in areas of permanent water (oases) or where water comes close to the surface. Here, the local pressure on natural resources can be intense. The remaining populations of large mammals have been greatly reduced by hunting for food and recreation. In recent years development projects have started in the deserts of Algeria and Tunisia using irrigated water pumped from underground aquifers. These schemes often lead to soil degradation and Soil salinity.
Researchers from Hacettepe University have reported that Saharan soil may have bio-available iron and also some essential macro and micro nutrient elements suitable for use as fertilizer for growing wheat.
A 2025 study sequenced individuals from Takarkori (7,000 YBP) and discovered that most of their ancestry was from an unknown ancestral North African lineage, related to the African admixture component found in . According to the study, the Takarkori people were distinct from both contemporary sub-Saharan Africans and non-Africans/Eurasians. They had "only a minor component of non-African ancestry" but did "not carry sub-Saharan African ancestry, suggesting that, contrary to previous interpretations, the Green Sahara was not a corridor connecting Northern and sub-Saharan Africa."Max Planck Institute. First ancient genomes from the Green Sahara deciphered. 4 February 2025 As per Johannes Krause of the Max Planck Institute, one of the authors of the study "The Takarkori lineage likely represents a remnant of the genetic diversity present in northern Africa between 50,000 and 20,000 years ago."
In the Central Sahara, engraved and painted rock art were created perhaps as early as 10,000 years ago, spanning the Bubaline Period, Kel Essuf Period, Round Head Period, Pastoral Period, Caballine Period, and Cameline Period. The Sahara was then a much wetter place than it is today. Over 30,000 of river such as National Geographic News , 17 June 2006. survive, with half found in the Tassili n'Ajjer in southeast Algeria. of , including Afrovenator, Jobaria and Ouranosaurus, have also been found here. The modern Sahara, though, is not lush in vegetation, except in the Nile Valley, at a few oasis, and in the northern highlands, where Mediterranean plants such as the olive tree are found to grow. Shifts in axial tilt increased temperatures and decreased precipitation, which caused an abrupt beginning of North Africa desertification about 5,400 years ago.
By 3400 BCE, the Sahara was as dry as it is today, due to reduced precipitation and higher temperatures resulting from a shift in Earth's orbit. As a result of this aridification, it became a largely impenetrable barrier to humans, with the remaining settlements mainly being concentrated around the numerous oases that dot the landscape. Little trade or commerce is known to have passed through the interior in subsequent periods, the only major exception being the Nile Valley. The Nile, however, was impassable at several cataracts, making trade and contact by boat difficult.
As areas where the Tichitt cultural tradition were present, Dhar Tichitt and Dhar Walata were occupied more frequently than Dhar Néma. Farming of crops (e.g., millet) may have been a feature of the Tichitt cultural tradition as early as 3rd millennium BCE in Dhar Tichitt.
As part of a broader trend of iron metallurgy developed in the West African Sahel amid 1st millennium BCE, iron items (350 BCE – 100 CE) were found at Dhar Tagant, iron metalworking and/or items (800 BCE – 400 BCE) were found at Dia Shoma and Walaldé, and the iron remnants (760 BCE – 400 BCE) found at Bou Khzama and Djiganyai. The iron materials that were found are evidence of iron metalworking at Dhar Tagant. In the late period of the Tichitt Tradition at Dhar Néma, tamed pearl millet was used to temper the tuyeres of an oval-shaped low shaft furnace; this furnace was one out of 16 iron furnaces located on elevated ground. Iron metallurgy may have developed before the second half of 1st millennium BCE, as indicated by pottery dated between 800 BCE and 200 BCE. At Dhar Walata and Dhar Tichitt, copper was also used.
After its decline in Mauritania, the Tichitt Tradition spread to the Middle Niger region (e.g., Méma, Macina, Dia Shoma, Jenne Jeno) of Mali where it developed into and persisted as Faïta Facies ceramics between 1300 BCE and 400 BCE among rammed earth architecture and iron metallurgy (which had developed after 900 BCE). Thereafter, the Ghana Empire developed in the 1st millennium CE.
The Libyco-Berber alphabet of the ancient Libyans of north Africa seems to have been based on Phoenician, and its descendant Tifinagh is still used today by the (Berber) Tuareg people of the central Sahara.
The Periplus of the Phoenician navigator Hanno, who lived sometime in the 5th century BCE, claims that he founded settlements along the Atlantic coast of Africa, possibly including the Western Sahara. The identification of the places discussed is controversial, and archeological confirmation is lacking.
Between the first century BCE and the fourth century CE, several Roman expeditions into the Sahara were conducted by groups of military and commercial units of Roman Empire.
The Beni Ḥassān and other nomadic Arab tribes dominated the Sanhaja Berber people tribes of the western Sahara after the Char Bouba war of the 17th century. As a result, Arabian culture and language came to dominate, and the Berber tribes underwent some Arabization.
The French took advantage of long-standing animosity between the Chaamba Arabs and the Tuareg. The newly raised Méhariste camel corps were originally recruited mainly from the Chaamba nomadic tribe. In 1902, the French penetrated the Hoggar Mountains and defeated Kel Ahaggar in the battle of Tit.
The French Colonial Empire was the dominant presence in the Sahara. It established regular air links from Toulouse (HQ of famed Aéropostale), to Oran and over the Hoggar to Timbuktu and West to Bamako and Dakar, as well as trans-Sahara bus services run by La Compagnie Transsaharienne (est. 1927). A remarkable film shot by famous aviator Captain René Wauthier in 1933 documents the first crossing by a large truck convoy from Algiers to Tchad, across the Sahara.
Egypt, under Muhammad Ali and his successors, conquered Nubia in 1820–22, founded Khartoum in 1823, and conquered Darfur in 1874. Egypt, including Sudan, became a British protectorate in 1882. Egypt and Britain lost control of the Sudan from 1882 to 1898 as a result of the Mahdist War. After its capture by British troops in 1898, the Sudan became an Anglo-Egyptian condominium.
Spain captured present-day Western Sahara after 1874, although Rio del Oro remained largely under Sahrawi people influence. In 1912, Italy captured parts of what was to be named Ottoman Libya from the Ottomans. To promote the Roman Catholic religion in the desert, Pope Pius IX appointed a delegate Apostolic of the Sahara and the Sudan in 1868; later in the 19th century his jurisdiction was reorganized into the Vicariate Apostolic of Sahara.
Most of the Saharan states achieved independence after World War II: Libya in 1951; Morocco, Sudan, and Tunisia in 1956; Chad, Mali, Mauritania, and Niger in 1960; and Algeria in 1962. Spain withdrew from Western Sahara in 1975, and it was partitioned between Mauritania and Morocco. Mauritania withdrew in 1979; Morocco continues to hold the territory (see Western Sahara conflict).
Tuareg people in Mali rebelled several times during the 20th century before finally forcing the Malian armed forces to withdraw below the line demarcating Azawad from southern Mali during the 2012 rebellion. Islamist rebels in the Sahara calling themselves al-Qaeda in the Islamic Maghreb have stepped up their violence in recent years.
In the post–World War II era, several mining and communities have developed to use the desert's natural resources. These include large deposits of petroleum and natural gas in Algeria and Libya, and large deposits of in Morocco and Western Sahara. Libya's Great Man-Made River is the world's largest irrigation project. The project uses a pipeline system that pumps fossil water from the Nubian Sandstone Aquifer System to cities in the populous Libyan northern Mediterranean coast including Tripoli and Benghazi.
A number of Trans-African highways have been proposed across the Sahara, including the Cairo–Dakar Highway along the Atlantic coast, the Trans-Sahara Highway from Algiers on the Mediterranean to Kano in Nigeria, the Tripoli – Cape Town Highway from Tripoli in Libya to N'Djamena in Chad, and the Cairo – Cape Town Highway which follows the Nile. Each of these highways is partially complete, with significant gaps and unpaved sections.
Evidence for cycles
Ecoregions
Flora and fauna
History
Kiffians
Tenerians
Tashwinat Mummy
Nubians
Egyptians
Tichitt culture
Phoenicians
Greeks
Garamantes
Islamic and Arabic expansion
Ottoman Turkish era
European colonialism
Breakup of the empires and afterwards
Peoples and languages
See also
Bibliography
External links
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