Deep in the Nigerian rainforest, there was once an immense kingdom surrounded by a huge earthen wall and moat. Built during the Middle Ages, this great African kingdom was larger than many contemporary kingdoms of its time such as Baghdad or Cairo or Rome. Yet, nothing of this vast kingdom survive today save for its fortification wall—a structure so enormous that its construction was a feat that was bigger in scale than that of the Great Pyramid of Giza. The BBC calls it “Africa's largest single monument”.

Photo credit: Jeremy Weate/Flickr