30 million. It could be just another statistic, another number in a blur of facts and figures that fly by our faces in a day. But this 30 million has a face. It is the face of the modern slave.

The Global Slavery Index 2013 has been released. It estimates that there are 30 million people held in bondage around the world: in the sex trade, domestic servants, farm workers, child soldiers. Of course, that is only an estimate, as slavery depends on secrecy to continue. China and Pakistan have the most slaves in terms of population, but if the numbers are adjusted for percentage of population living in slavery, the African country of Mauritania is slavery’s modern outpost. Anywhere between 10 to 20 percent of Mauritania’s population are slaves, despite the fact that slavery has been illegal there since 1981.

CNN’s John D. Sutter went to Mauritania to document slavery there. In “Slavery’s Last Stronghold,” Sutter met slave owners, former slave owners, slaves and abolitionists. One man, Abdel Nasser Ould Ethmane, chose his first slave when he was 7 years old, but has since e an abolitionist.

Abdel was careful to say his family never beat his slave, Yebawa Ould Keihel. Family members did, however, force him to tend their herds of goats and camels, out in the deserts of central Mauritania, without pay. At the time, Abdel told us, he didn’t feel guilty. In fact, he and the other children in their nomadic group, which followed water from one anonymous area of the Sahara to the next, openly taunted the slaves who served them. When it rained on the Tagant plateau, slaves like Yebawa had to hold up the edges of the master-family’s tent to prevent water from leaking in, he told us. Abdel recalls hearing the slaves’ teeth chattering through the cold desert nights — and mocking this “teeth music” with his slave-owning friends.

“Here they were standing up, protecting us, and we pletely unconscious (and) ignorant,” Abdel said. “This was actually quite innocent because, for us, slavery was really a natural state. One must really have in mind that when one is born into a certain environment, it is considered the right one — just and fair.”

What changed Abdel’s mind about what is “just and fair?” He was sent away to school at the age of 12, and discovered a love of reading.

Hesitant at first, Abdel soon dove into every book he could find. He started with ic books like “Asterix.” It wasn’t long before he was picking up volumes about the French Revolution.

In a book on the subject of human rights, pulled from the library’s shelves almost at random, Abdel found the idea that would alter his life forever:

Men are born and remain free and equal in rights.

Abdel read the line again and again.

“I started to ask myself if lies ing out of this book,” he told us, “or if they were ing out of my very own culture.”

What is there to say? Owning another human being is not possible; we are all born free, equal in rights, made in God’s image and likeness. And yet, at least 30 million of our brothers and sisters are enslaved. It seems cliché to say we must do something, but we must. Every person on earth must know what Abdel discovered at the age of 12, in a boarding school library: Men are born and remain free and equal in rights.

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