But slaves are hard to count in a country whose landscape is covered by the drought-wrung, ever-expanding Sahara, even as it is slaves who perform the desert economy's most demanding work: When Lam introduces me to a recently escaped family of slaves, I learn that the young children in the family had been made to herd camels - an especially arduous job in this harsh climate.

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A holistic fight against social injustice It is Ramadan when I visit and the temperature in Nouakchott has risen to its typical summer high of somewhere around 43 degrees.

Although I have fasted through plenty of holy months since I converted to Islam some 20 years ago, the extreme heat here is making me realise that I may need water.

When I got off the plane at the Nouakchott International Airport, I had just flown over miles and miles of the Sahara Desert, looking out of the airplane window at so much endless, timeless sand that the dunes seemed, to my mind, like whitecaps on an ocean.

When I got off the plane at the Nouakchott International Airport, I was landing in a country wholly apart from the place from which I'd come.

In 1981, the government became the last in the world to abolish slavery, but slave-owning did not become a punishable offence until 2007.

In August of this year, thanks to the efforts of anti-slavery activists such as Lam, the country doubled the prison term for offenders from 10 to 20 years, and criminalised 10 other forms of slavery, including forced marriage.

According to the Walk Free Foundation's Global Slavery Index, Mauritania, with its population of just 3.8 million, has between 140,000 and 160,000 slaves.

That's about four percent of the population, and it is, according to many organisations, including Lam's, a rather conservative estimate: they point to a figure closer to 15 percent.

She was dressed, on the day I met her, in a powder blue mulaffa - the traditional dress of Mauritania.

With her soft voice and beatific smile, she might best be described as a presence.

Lam notes, however, that to date, only one slaveholder has been conclusively prosecuted for owning slaves, in November 2011, after two boys - aged 10 and 12 - escaped confinement and turned to the Initiative for the Resurgence of the Abolitionist Movement (the IRA).