What is the largest desert in the world?

Antarctica, which catches people out because a desert is defined by how little precipitation falls, not by heat or sand. The interior receives the equivalent of about 50 millimetres of water a year, less than much of the Sahara, and parts of the McMurdo Dry Valleys may not have seen rain for two million years.

That makes it the largest desert on Earth at around 14 million square kilometres, with the Arctic second and the Sahara only third. Sand deserts are also the exception rather than the rule - only about a fifth of the world's desert area is dunes; the rest is rock, gravel and ice.

The dryness is why Antarctica preserves things so well. Scott's huts still contain legible labels on tins of food and stacked hay; meteorites accumulate on the ice for tens of thousands of years and are easy to spot against it, which is why most of the world's collected meteorites come from there; and the ice itself is a layered archive of the atmosphere, with bubbles of ancient air that let us read carbon dioxide levels back 800,000 years.