At its center is Ar-Rub' al-Khali (The Empty Quarter), one of the largest continuous bodies of sand in the world.

[4] Gazelles, oryx, sand cats, and spiny-tailed lizards are just some of the desert-adapted species that survive in this extreme environment, which features everything from red dunes to deadly quicksand.

[1] The Rub' al-Khali desert is a sedimentary basin stretching along a south-west to north-east axis across the Arabian Shelf.

[5] At an altitude of 1,000 metres (3,300 ft), rock landscapes yield to the Rub' al-Khali, a vast stretch of sand whose extreme southern point crosses the center of Yemen.

The sand overlies gravel or gypsum plains and the dunes reach maximum heights of up to 250 m (820 ft).

[citation needed] The Tuwaiq escarpment is an 800 km (500 mi) arc that includes limestone cliffs, plateaus, and canyons.

Thus clear-sky conditions with plenty of sunshine prevail over the region throughout the year, and cloudy periods are infrequent.

Very few trees are found except at the outer margin (typically Acacia ehrenbergiana, Vachellia tortilis, Balanites aegyptiaca, Ziziphus spina-christi, Olea europaea, Cupressus sempervirens, Senegalia senegal, Pinus nigra, Aerva javanica, Populus alba, Populus nigra, Salix alba, Quercus coccifera, Quercus ilex, Quercus pubescens, Pistacia terebinthus, Sclerocarya birrea, Adansonia digitata, Ficus salicifolia, Pinus pinea, Ceratonia siliqua, Prunus amygdalus, Corylus avellana, Pinus halepensis, Laurus nobilis, Phoenix canariensis, Phoenix dactylifera, Arbutus unedo, Juniperus communis, Faidherbia albida, Borassus aethiopum, Vitellaria paradoxa, Afzelia africana, Kigelia africana, and Prosopis cineraria).

[8] Native mammals include the Arabian oryx (Oryx leucoryx), sand gazelle (Gazella marica), mountain gazelle (G. gazella), Nubian ibex (Capra nubiana), Arabian wolf (Canis lupus arabs), striped hyaena (Hyaena hyaena), caracal (Caracal caracal), sand cat (Felis margarita), red fox (Vulpes vulpes), and Cape hare (Lepus capensis).

[citation needed] Threats to the ecoregion include overgrazing by livestock and feral camels and goats, wildlife poaching, and damage to vegetation by off-road driving.

In the UAE, the sand gazelle and Arabian oryx are threatened, and honey badgers, jackals, and striped hyaenas already extirpated.

[1] Saudi Arabia has established a system of reserves overseen by the National Commission for Wildlife Conservation and Development (NCWCD).