Arabia is mainly a great plateau of ancient crystalline rock, largely covered with limestone and sandstone. It rises steeply from the narrow Red Sea coastal plain, achieving its greatest height (c.12,000 ft/3,700 m) in SW Arabia, and slopes gently E to the Persian Gulf; the Oman Mts., SE Arabia, rise to c.10,000 ft (3,000 m). The coastal mountains catch what little moisture is carried by the dry winds that cross Arabia, making the interior so arid (4 in./10 cm annual precipitation) that there is not a single perennial stream; thus, large areas lack water. The basin-shaped interior consists of alternating steppe and desert landscape; the Nafud desert in the north is connected with the great Rub al-Khali in the south (one of the world's largest sand deserts) by the Dahna, a narrow sand corridor.
Extensive and varied agriculture (coffee, grains, fruits) exists only in SW Arabia, particularly in Yemen, where high coastal mountains intercept the moist southwest monsoon winds during the summer. The northeast coast of Oman has a climate similar to that of Yemen, but in most of Arabia rainfall occurs only in winter. The coastal lands, however, are much more humid than the interior; fog and dew are common. Desalination plants supply much of the population's drinking needs. Osmosis distillation processes take brackish underground water and make it useful for agriculture and industry.