it shares with the An Nafud the longitudinal structure of sand dunes. The Ad Dahna also furnishes the Bedouin with winter and spring pasture, although water is scarcer than in the An Nafud. The southern portion of the Ad Dahna curves westward following the arc of the Jabal Tuwayq. At its southern end, it merges with the Rub' al Khali, one of the truly forbidding sand deserts in the world and, until the 1950s, one of the least explored. The topography of this huge area, covering more than 550,000 square kilometers, is varied. In the west, the elevation is about 600 meters, and the sand is fine and soft; in the east, the elevation drops to about 180 meters, and much of the surface is covered by relatively stable sand sheets and salt flats. In places, particularly in the east, longitudinal sand dunes prevail; elsewhere sand mountains as much as 300 meters in height form complex patterns. Most of the area is totally waterless and uninhabited except for the few wandering Bedouin tribes. Caves Beneath the harsh deserts of Saudi Arabia lie dark chambers and complex mazes filled with crystalline structures, stalactites and stalagmites. The limestone floor of the Summan plateau, a karst area to the east of the Dahna sands, is riddled with such caves, known locally as Dahls. Some have tiny entrances which open into caves, others lead into a maze of passages which can be several kilometers long. Local Bedouin have long known of these caves and some were used as water supplies. They were first systematically studied in 1981, and later explored and reported by the Saudi Geological Survey. The environment and the Gulf War The Persian Gulf War of 1991 brought serious environmental damage to the region. The world's largest oil spill, estimated at as much as , fouled gulf waters and the coastal areas of Kuwait, Iran, and much of Saudi Arabia's Persian Gulf shoreline. In some of the sections of the Saudi coast that sustained the worst damage, sediments were found to contain 7% oil. The shallow areas affected normally provide feeding grounds for birds, and feeding and nursery areas for fish and shrimp. Because the plants and animals of the sea floor are the basis of the food chain, damage to the shoreline has consequences for the whole shallow-water ecosystem, including the multimillion-dollar Saudi fisheries industry. The spill had a severe impact