The vast barrenness of the Libyan Desert stretches from the Nile westward across Egypt and northern Sudan to Tripolitania in Libya. For the ancient Egyptians it was the realm of the afterlife, overseen by Osiris, a place of fear and dread. According to the historian Herodotus, writing in the fifth century BC, a huge sandstorm once swallowed up an entire army of invading Persians there without a trace. To modern Egyptians, the Libyan Desert is increasingly a realm of hope—a hope based on extensive irrigation schemes to increase agricultural land and relieve crowding in the Nile Valley.

At the beginning of the 20th century, the desert to which Egypt now turns with such energy was almost entirely unmapped. Even among the sprinkling of nomads who braved its harshness, knowledge was fragmentary at best. Large areas were entirely avoided due to lack of water or daunting terrain. Legends of the desert echoed in the minds of the inhabitants of the Egyptian oases: tales of spirits and shadowy raiders, of lost oases and lost treasure.

The legends surrounded one of these "lost oases" in particular: Its name was Zerzura. For three decades at the opening of the century, that name tugged at the imagination of every explorer of the Libyan Desert. Its existence and location were debated in the distinguished pages of the Journal of the Royal Geographical Society under such titles as "The Zerzura Problem," "Problems of the Libyan Desert" and "Lost Oases of the Libyan Desert."

The searchers for Zerzura would indeed find treasure, but of new knowledge, not gold. They would discover evidence of earlier peoples who had lived in the region before the climate changed, and they would develop new ways of traveling across the great oceans of dunes. One of them would outline the science of dune formation, and this knowledge in turn would prove to have literally otherworldly applications.

In Arabic, the word zerzura can connote a location populated by starlings or other little birds. The oldest record of a location with this name is found in a document dating to the mid-13th century, written by the amir Osman al-Nabulsi, regional administrator of the Fayyum. An anonymous 15th-century Arabic treasure-hunters' guide, Kitab al-Kdnuz, "The Book of Hidden Pearls," describes Zerzura as a whitewashed city of the desert on whose gate is carved a bird. The treasure seeker is advised to "take with your hand the key in the beak of the bird, then open the door of the city. Enter, and there you will find great riches...."

The first European reference to Zerzura is in an 1835 account by the English Egyptologist John Gardner Wilkinson, based on a report by an Arab who said he had found the oasis while searching for a lost camel. Placed five days west of the track connecting the oases of Farafra and Bahariya, the "Oasis called Wadee Zerzoora" abounded "in palms, with springs, and some ruins of uncertain date." Although tales of secret desert locales found by searchers for stray camels were common enough, Wilkinson's account was bolstered when later explorers found a number of previously unknown oases that had been named in his account along with Zerzura. But they did not find Zerzura itself.

Five days' travel west of the Farafra-Bahariya track would place Zerzura within or near the eastern boundary of the Great Sand Sea. Although the existence of this large mass of sand dunes south of Siwa Oasis had been known at least since the time of Herodotus, how far it extended into the interior of the country was not known to outsiders before the 1874 journey of the German explorer Gerhard Rohlfs. Traveling west from Dakhla Oasis, he penetrated the eastern edge of the sands and, finding his westward progress blocked by great dunes running north and south, he turned northward to Siwa, making good time on the hardened "roads" between the dunes. At a place he named Regenfeld because of a chance rainstorm, he left a cairn of stones to guide future explorers. Decades would pass before any ventured forth.

From 1909 to 1911, W. J. Harding King made several journeys along an old trade road that ran southwest from Dakhla and which, he reasoned, must have had watering points along its length. He had heard stories of Zerzura, including one that described it as an abandoned city with palm and olive groves. After seeing a flight of small birds enter Dakhla from the southwest and finding freshly eaten olives in their stomachs, Harding King became convinced he would find the lost "olive" oasis southwest of Dakhla. He retraced 320 kilometers (200 mi) of the old road but found no abandoned desert city.

Six years later John Ball, director of the Egypt Survey Department, used Dakhla as a base for his initial attempt to find the lost oasis. Traveling by car, he advanced 320 kilometers west from Dakhla, rounding the southeast extremity of the Great Sand Sea. Some 160 kilometers (100 mi) from Dakhla, he did find a cache of clay jars—some raiding party's water depot—but no lost oasis.

In the years 1921-1924, Lieutenant Colonel N. B. de Lancey Forth made several journeys by camel into the Great Sand Sea. To avoid getting lost in the dunes, Forth "marked our route with thin palm branches placed in conspicuous positions, at distances varying from a few hundred yards to a mile or more apart." He made observations of dune movement and found important evidence of Stone Age habitation within the sands: flint implements, remains of ancient camp-fires and fragments of petrified ostrich and oyster shells. These provided insight into the climatic changes that had occurred over the millennia—but again, no forgotten oasis revealed itself.

The first breakthrough in Libyan Desert exploration did not happen until 1923. It was the result of an extraordinary journey by an equally extraordinary man: Ahmed Mohamed Hassanein Bey. Esteemed by his contemporaries, Hassanein Bey blended his Oxford education with a heartfelt loyalty to the customs and beliefs of his native Egypt. Traveling by camel with only a handful of companions, Hassanein Bey based his hopes for safe passage on his extensive friendships among the Senussi, a Sufi brotherhood whose main center was at Kufra, and on the tribal peace they had induced in the region. Setting out from Saloum on the Mediterranean coast in early January, he followed caravan routes through the oases of Siwa, Jaghbub and Jalo before reaching Kufra Oasis on April 1, 1923. On April 18th, after reciting the Fatihah and praying with his men, he led his refitted caravan south from Kufra into the unknown.

High daytime temperatures necessitated long hours of night travel over rough terrain. Men and animals tired rapidly. In the early hours of April 24, the column was "having a hard time of it crossing the high steep sand-dunes when suddenly mountains rose before us like mediaeval castles half hidden in the mist." Letting the caravan go on, Hassanein Bey sat for half an hour atop a dune savoring "the outstanding moment of the whole journey." One hundred and eleven days after leaving Saloum, he had located the "lost" mountain oases of Arkenu and 'Uwaynat. Unlike the artesian-fed oases scattered across the desert, Arkenu and 'Uwaynat, the highest mountains in the Libyan Desert, were found to be sustained by rainfall that collected in caves. After exploring the oases and visiting with the Guraan tribesmen at 'Uwaynat, Hassanein Bey set out for El Fasher in the Sudan, reaching it at the end of May. His remarkable journey had covered 3500 kilometers (2200 mi).

Significant contributions to geographic knowledge resulted from Hassanein Bey's careful measurements. He fixed the positions of Arkenu and 'Uwaynat, previously known to the outside world only by rumor, and determined more accurate positions for a number of other locations. The map prepared by the Egypt Survey Department from his account of his journey gave the most accurate overview of the Libyan Desert of the time. Vast areas, however, remained blank. The Gilf Kebir, the Qattara Depression, the Selima Sand Sheet and most of the Great Sand Sea remained unknown.

Another Egyptian, Prince Kemal el Din, played a major role in filling those blanks. The grandson of Khedive Ismail, he brought an active mind, adventurous spirit and—importantly—deep pockets to the search for Zerzura. To help extend the speed and range of his expedition, he commissioned manufacture of a group of tracked vehicles from Citroën. Using these, he and Ball made several journeys together. In 1923 they headed west-southwest from Dakhla to further explore the extent of the Sand Sea and to look for a lost oasis that would link Dakhla with oases to the west. In 1924 they drove into the Sand Sea. In 1925 they were the first to reach Jabal 'Uwaynat from the Nile Valley, traveling via Bir Terfawi in the Selima Sand Sheet, where they found "absolutely nothing to be looked at...but the eternal smooth curving outlines of the sand repeating themselves with geometric regularity." In 1926 the prince was also the first to reach Jabal 'Uwaynat from the direction of Dakhla. In the process, he discovered the Gilf Kebir, a plateau the size of Switzerland, ringed by cliffs, between the Sand Sea and 'Uwaynat.

Kemal el Din and Ball's journeys mapped large areas in the southern and western regions of the Egyptian portion of the Libyan Desert. Their clarification of the extent of the Sand Sea, and the prince's discovery of the Gilf Kebir, provided Egypt with a clear picture of this natural defensive barrier of huge dunes and sheer cliffs. Utilizing their findings, Ball produced a contour map of the Libyan Desert and also mapped the static water table beneath it.

The late 1920's saw the debate over Zerzura reach fever pitch. A number of articles about the "rather complex subject" aired strongly held viewpoints. Based on evidence he had found on his travels, de Lancey Forth thought the oasis could be found west-northwest of Dakhla. Ball felt that it could possibly be found within the Sand Sea, more likely to the west of Bir Terfawi. Harding King clung to his idea of a rain-fed "olive" oasis between Jabal 'Uwaynat and Dakhla. Douglas Newbold of the Sudan Service placed it west of the Selima Oasis. The Hungarian Count László Almásy believed it would be found in the area of the Gilf Kebir.

While the debate raged, advances were being made in the technology of desert travel. An enterprising group of English officers, including Ralph Bagnold, found they could traverse dunes in the lightweight Ford Model A by lowering the pressure in the tires. They developed "sand ladders" of wire and bamboo to help vehicles run atop soft sand and modified the cars' bodies to make them expedition-efficient. By developing a system for conserving radiator water, they enormously extended the potential range of expeditions. These improvements allowed them mobility unattainable with the heavier Citroëns, opening the Sand Sea to broader exploration. And soon aircraft began to accompany expeditions.

This combination of technology and intellectual fervor was brought to bear on the desert's remaining secrets. In 1930, Bagnold and his companions traveled from Farafra to Jabal 'Uwaynat, penetrating the Sand Sea to an unprecedented depth. But they found that, despite new innovations, travel there remained perilous. When their cars stuck in a dead end amid the dunes, "no one made any effort to move." They lay on the sand "dreaming pleasantly, ...rather interested, in a detached impersonal way, at our own apathy." This dehydration-induced stupor could easily have had a fatal outcome, but "someone had a bright idea. We broke into the water supply and had an extra pint all round. The effect was marvelous; the water acted as a stimulating drug." In two hours they freed their vehicles and they began to retrace their tracks. After 20 kilometers (12 mi) they found a gap in the dunes that they had missed earlier; it opened onto a broad avenue of firm ground.

In 1932 an expedition including a young English baronet, Robert Clayton East-Clayton, Count Almásy, Colonel Pat Clayton of the Survey Department and Wing Commander H. W. G. J. Penderel made an aerial sighting of two green valleys in the Gilf Kebir. Was Zerzura found at last? Attempts to reach the valleys overland failed when gasoline ran short. At one point Penderel and Pat Clayton found themselves stranded. Undaunted, they "brewed a cup of tea from radiator water. Although the water was darker than the tea," Clayton related, "I have never tasted a drink nearly so good."

Their sighting created a sensation, and in early 1933, two separate expeditions returned to the Gilf Kebir. The first was that of Pat Clayton and Lady Dorothy Clayton, the widow of Sir Robert. Meeting at Kufra, they explored the western face of the Gilf Kebir and located the two green valleys, Wadi Hamra and Wadi Abd el Melik. A short time later, Almásy and Penderel explored the eastern face of the Gilf Kebir. They found a third green valley, Wadi Talh, as well as the Aqaba Pass, which cuts through the plateau from east to west. (During World War II Almásy used this passage to evade Allied patrols while smuggling the German spy "Rebecca" to the outskirts of Dhakla. Almásy's exploits were the chief inspiration for the novel The English Patient, by Michael Ondaatje, and the 1996 Oscar-winning film of the same name.)

Ralph Bagnold and his companions also undertook an incredible odyssey in 1932. Traversing 6000 direct kilometers (3700 mi), 9600 kilometers total (6000 mi), they crisscrossed the desert from Cairo to northern Chad, ending at Wadi Haifa on the Nile in Sudan. Although Bagnold wasn't as caught up in "Zerzura fever" as many of his colleagues, the search lingered in the back of his mind as he set out, "in case, by the merest chance....!" The extent of his journey revolutionized the scope of desert travel.

The explorations of 1932 and 1933 filled the last major blanks on the Libyan Desert map, the Qattara Depression having been placed by George Walpole in 1926. Prior to his death in 1932, Prince Kemal el Din had written that "the one remaining unaccomplished journey in Africa" was that from Kufra to Siwa through the Sand Sea. A first east-to-west crossing of the Sand Sea was accomplished with some difficulty in December of 1932 by Pat Clayton. Early in 1933, he and Lady Dorothy Clayton made the first direct passage from Kufra to Siwa with relative ease.

What remained unresolved was the debate over whether Zerzura had in fact been found. A century earlier, Wilkinson had been told by nomads at Dakhla of a mysterious oasis formed by three green valleys. But was this different from the "Wadee Zerzoura" that lay to the west of Farafra with its "ruins of uncertain date"? Perhaps the green valleys of the Gilf Kebir were the Zerzura of legend. Perhaps Zerzura was a location swallowed by the desert like the Persian army. Or perhaps it had never existed outside men's imaginations.

Nonetheless, the energy and intellect that was applied to Zerzura's discovery produced a wealth of knowledge. The barren vastness of the Libyan Desert was mapped, as was the water table below it—vital knowledge for the Egyptian nation in the modern era. Technical advances provided a foundation of knowledge for future motorized desert travel—knowledge that a number of the English explorers would utilize in the Long Range Desert Group in World War II. (Ralph Bagnold reportedly proposed formation of the unit to General Sir Archibald Wavell, Commander in Chief of Middle East Land Forces, with the words, "How about some piracy on the high desert?")

It was also Bagnold who almost single-handedly made some of the most important intellectual contributions that sprang from the search for Zerzura. Throughout his journeys, he had been fascinated by the shapes, consistency and movement of sand dunes. He wondered how their surfaces seemed to flow under foot in a strong wind; how they kept their shapes as they moved, even repairing damage done to them; how they grew rather than being dispersed by the wind and, in some areas, bred "babies" that ran ahead of the parent dune. Since the Libyan Desert was virtually free of foliage and habitation, dune movement was unimpeded, and Bagnold had a perfect "laboratory" for field observations.

After his days of active exploration were over, Bagnold set up experiments to try to find answers to his questions. From 1935 to 1939, he worked at his homemade wind tunnel in England, with a return to the Gilf Kebir in 1938 for additional field study. His findings were published in 1941 by Methuen under the title The Physics of Blown Sand and Desert Dunes, which remains the standard in the field. Its principles also formed a basis for the study of wind erosion of agricultural land. Decades later, Bagnold also found them applicable to predicting the movement of sediment in rivers. The rapidly developing oil business in the Middle East was an early beneficiary of Bagnold's insights into how dune movement could threaten drilling rigs and how such movement might be channeled.

The search for Zerzura also benefited exploration of a much more distant barren landscape. In 1977, the US National Aeronautics and Space Administration (NASA) succeeded in landing an unmanned probe on the surface of Mars. Photos taken by the probe showed what appeared to be sand dunes similar to those of Earth. In an effort to better understand these Martian dunes and any hazard they might pose to the probe, NASA called on the services of an 81-year-old desert explorer and seeker of Zerzura, Ralph Bagnold.