Why are millions of people still trapped in slavery?

Slavery is illegal in every corner of the world, yet an estimated 21 million people are in thraldom globally. To put that number in perspective, there are more people enslaved in contemporary society than at the height of the transatlantic slave trade. How is this possible? And would you even recognise a modern slave if you saw one in the street?

Original source: The Guardian. 
Posted on 31 July 2017.