Very interesting work on African geography and development by Nathan Nunn & Diego Puga:
In Africa, between 1400 and 1900, four simultaneous slave trades, across the Atlantic, the Sahara Desert, the Red Sea and the Indian Ocean, led to the forced migration of as many as 18m people. The economies they left behind were devastated: political institutions collapsed, and societies fragmented.
For African people fleeing this slave trade over the centuries, rugged terrain was a positive advantage. Enslavement often took place through raids by one group on another, and hills and mountains provided plenty of lookout posts and hiding places (caves, for example) for those trying to escape. In general, countries with flatter, more passable terrain lost more of their population to the traders.
Today, however, that same geographical ruggedness is an economic handicap, making it expensive to transport goods to port; raising the cost of irrigating and farming the land; and simply making it more expensive to do business… hundreds of years of flight from the slave trade has left the African population disproportionately concentrated in hilly areas…
So the slave trades left a doubly toxic economic legacy in Africa: not only did they devastate the population in many areas, with long-lasting impacts which still persist centuries later; they also left the African population concentrated in areas which make contemporary economic development harder.