Rubʿ al-Khali takes up a fifth of the Arabian Peninsula and holds about as much sand as the Sahara. The desert is 1,000 kilometres (620 miles) long and 500 kilometres (310 miles) wide. The sand dunes can reach heights of 250 metres and are a reddish-orange colour due to the presence of feldspar. There are brackish salt flats in some areas. The annual rainfall in the area is less than 30 millimetres (1.2 inches), while the average daily maximum temperature is about 47 °C (117 °F) and has reached as high as 56 °C (133 °F).
Along the middle of the desert are raised and hardened areas of calcium carbonate, gypsum, marl, or clay that show that they were once the site of shallow lakes. The lakes existed in two different periods: 6,000 to 5,000 years ago and 3,000 to 2,000 years ago. The lakes likely formed as a result of an enormous amount of rainfall, similar to present-day monsoons. While the rains likely lasted for only a few years, there are lakes in the Mundafen area in the southwest of the Rub' al Khali that seem to have lasted longer, up to 800 years.
Fossilised remains show several animal species used to use the lakes. Remains of hippopotamus, water buffalo, and long-horned cattle have been identified, as well as small snails, ostracods, and some freshwater clams. The presence of calcium carbonate and opal phytoliths indicate that plants and algae once flourished. While no human remains have been discovered, flint tools dating back 3,000 to 2,000 years ago have been found.
Photograph: George Steinmetz