The first of five clues. Each one below gives a little more away.
Six thousand years ago I was green, with lakes in me and hippos in the lakes.
My dust crosses an ocean and feeds a rainforest.
I am roughly the size of the United States.
Only a small part of me is the sand dune everybody pictures.
The great desert across the north of Africa.
Sahara
Around 180 million tonnes of its dust blow over the Atlantic every year, and the phosphorus in that dust is a large part of what keeps the Amazon growing.
For several thousand years after the last ice age the Sahara was green — savannah with lakes, crocodiles, hippos and human settlements, recorded in rock paintings of cattle and swimmers deep in what is now bare desert. A slow wobble in the Earth's orbit weakened the monsoon and the whole system collapsed within a few centuries. Only about a fifth of it is sand dunes; most is rock and gravel. Its dust matters far away: roughly 180 million tonnes cross the Atlantic each year, and the phosphorus in it fertilises the Amazon, which loses the same nutrient to rainfall.
Reading five clues at once is not the game. In the real thing they arrive one at a time and every wrong guess costs an hour. Play mystery #50 · today's mystery · the vault
Previous: mystery #49. Next: mystery #51. Or try #27, #28, #29, #30, #31, #32.