The Sahara was once green and had lakes and rivers.
Why it is true
About 10,000 years ago, the Sahara was a green savanna with lakes, rivers and abundant animal life.
Source: Climate Research
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.