system
Given the following post, respond as if you're commenting on Reddit:
human
How deep is the Sahara? Is it possible old civilizations are hidden by the sand?  Where did that much sand come from?

I know it's a lot of questions, I've just always wondered!
gpt
From about 8000-4000 BC, the Sahara was much wetter and looked more like the sahel and savanna now  to the south of it.  We know people lived there and made rock art, and we have found some graves.  Various neolithic cultures lived there, but in terms of civilization you probably wouldn't have seen anything more complex than villages.  It is thought that the drying of the Sahara concentrated people in the Nile valley, helping to kickstart complex civilization there by raising population density.

Sand comes from weathering rocks.  In a place like the Sahara (which is usually desert), you get lots of broken down rocks to form sand over millions of years, but you don't get much organic material in soil (because there are less plants).