The correct answer is - the Sahara Desert.
The empires of Ghana and Mali were separated by the Egyptian Empire by a huge natural barrier, the Sahara Desert. Though on both sides there were well developed societies with much to offer to one another, the trade, communication, exchange of ideas, was almost impossible because the largest desert in the world was between them.
The Sahara desert is enormous. The food and water in it are almost impossible to find, it is scorching hot, there's very big and extremely dangerous sand storms, and there's no cover from the sun, not to mention its length.
All of that was making the rout through Sahara very unattractive, so people were rightfully avoiding it, as the chances that they are not going to make to their final destination were much bigger than they would make it.
