The largest desert in the world is the interior of Antarctica. This has an area of about 14,000,000 km2 (5,400,000 sq mi). Rainfall in the Antarctic interior is extremely low, down to 20 millimeters (0.8 in) per year. This is partially because it is so cold that all liquid water quickly freezes solid and joins the ice pack.
The vast majority of liquid lakes in Antarctica are more than a mile below the surface. Some of these lakes are actually lower than freezing temperature, but remain liquid due to the great pressure above. Antarctica has only two major rivers, the Alph River and Onyx River. The latter is the longest in Antarctica, with a length of just 30 km (18 mi). By comparison, the Nile River is 6,650 km (4,135 mi) in length. Antarctica is not only the largest desert in the world, but also one of the oldest large deserts, forming 23 million years ago.
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But when most people think of desert, they think of hot. The next two largest deserts in the world -- the Sahara Desert in Africa and the Arabian Desert in the Middle East -- are both very dry and very hot. The Sahara Desert is the largest hot desert on Earth, and the hottest sustained temperature ever recorded on Earth was in the Sahara Desert, in September 1922 in Libya. The temperature was 58 °C (136 °F). Few large animals, camels and humans being two of them, can survive at such temperatures and dryness.
The Sahara Desert and the Arabian Desert have areas of 9,100,000 km2 (3,320,000 sq mi) and 2,330,000 km2 (900,000 sq mi), respectively. The Sahara Desert covers ten countries (Algeria, Chad, Egypt, Libya, Mali, Niger, Mauritania, Morocco, Sudan, and Tunisia), while the Arabian Desert covers another ten (Saudi Arabia, Jordan, Iraq, Iran, Kuwait, Qatar, United Arab Emirates, Oman and Yemen). The largest concentration of proven oil reserves in the world is found in the Arabian Desert.
Although today's deserts seem big, the largest desert of all time on the planet was the interior of Pangaea, a super-continent which existed from about 300 - 150 million years ago. The desert there might have been as much as seven times larger than the Antarctic Desert today. Most of the interior of the continent would have been uninhabitable to many forms of life.
The largest desert in the solar system is probably the surface of Venus. Venus' atmosphere has only .002% water vapor. The Earth's has about 1%. Venus' surface has temperatures exceeding 460 °C (860 °F), hotter than most broilers.