What is the largest hot desert in the world?

The Sahara, about nine million square kilometres - roughly the size of the United States, and the largest hot desert though not the largest desert, since Antarctica and the Arctic are both bigger by the definition that counts precipitation rather than temperature.

It has not always been desert and will not stay one. The Sahara alternates between arid and green on a cycle of roughly 20,000 years, driven by wobbles in the Earth's axis that shift the monsoon north and south. As recently as 6,000 years ago it held lakes, grassland, hippos and cattle herders, and there is rock art of swimmers in what is now some of the driest ground on the planet. It will green again, and not for tens of thousands of years.

It also fertilises the Amazon. Something like 27 million tonnes of Saharan dust cross the Atlantic each year, and a significant share falls on the rainforest carrying phosphorus that its heavily leached soils are short of. Much of that dust comes from the Bodele Depression in Chad, which is the dried bed of a vast ancient lake - so a rainforest on one continent is partly fed by the remains of a lake on another.