Where in the world does slavery still occur at the largest scale?

Today up to 600,000 Mauritanians, or 20% of the population, are still enslaved, mainly the "black Moors" of Haratin people. Though legally abolished in 1981, slavery did not become a crime until 2007, and since then only one person has been prosecuted for owning slaves. The issue of slavery is still powerful theme in the country's political debate.