Known of since the time of Herodotus, German explorer Gerhard Rohlfs was the first European to record the scale of the Great Sand Sea. He also gave it the name Sea. It blocked his way as he tried to go from Dakhla in Egypt to Kufra in Libya in 1874. In this same expedition he discovered Djarra Cave. Coming across the dune sea and with no real idea how far it stretched, Rohlfs followed the spaces between the parallel dunes in the general direction of Siwa
. After 18 days his camels had started to die from lack of water and the expedition was only saved by a fortuitous rainstorm. The site of the life-saving storm was named Regenfeld ('Rainfield') and is still marked by a cairn. There was originally a message in a bottle but that was removed by Hassanein Bey in 1923. He, in turn, left a message but now all messages and original water tanks and bottles have been removed by later visitors.
Around 40 years later other expeditions followed in Rohlfs' footsteps. John Ball approached the Great Sand Sea from Dakhla, by car, and skirted the southeastern edge on his first trip. He then returned and, joined by Prince Kamal al-Din, found the cairn Rohlfs had left at Regenfeld. It had survived due to being built at the foot of one of the giant whale-back dunes. Hassanein Bey followed and the story of his epic 3550km camel journey from Libya to Sudan through the Great Sand Sea can be read in The Lost Oases. Colonel de Lancey Forth also made two trips- the first traced Rohlfs' route and the second approached the Great Sand Sea from Siwa. He found ancient campfire sites with ostrich eggshells and flint knives from when the Sahara was lush and green. Bagnold and the Long Range Desert Group used Model-T Fords to explore the dune sea. They followed the corridors between the dunes, some of which they described as being over 150m high, and reached Siwa. In the 1940s, the north-south track was marked with rubbish, petrol cans and stones in the hope it would become a permanent route. It was marked on the Egyptian Desert Survey map by P.A. Clayton who was also the first to cross the Great Sand Sea by car from east to west. The route was in active use up till WW2 when both the British and German armies sent covert groups into the desert to report on each other's activities.