Last September, I boarded a rescue boat run by the medical charity Médecins Sans Frontières. My intention was to write about the two deadliest stretches of the mass-migration route from Africa into Europe—the Sahara Desert and the Mediterranean Sea. We left Malta at dawn, and, after several days of searching for boats in distress near the coast of Libya, the crew rescued three hundred and fifty-five migrants out of disposable inflatable dinghies. The boats had left from beaches near Tripoli with only enough fuel to reach international waters; without N.G.O.s carrying out search-and-rescue operations in the area, many of the migrants would have drowned.

The journey to Italy would take two and a half days. I walked around the ship, speaking to migrants about why they’d left home. Most were eager to talk about the horrors that they had endured in Libya, but on the lower deck I noticed a group of teen-age Nigerian girls, sitting together in silence. I sat down nearby and struck up a conversation with one of them, a girl named Blessing. She confided that she had been kidnapped, but gave no other details. The girl next to her stayed quiet, listening, as tears streamed down her cheeks.

By the time we reached the port of Messina, on the eastern coast of Sicily, I had begun to understand that criminal networks have co-opted sea rescues as a reliable means of delivering underage African girls to Europe’s prostitution market. Italy is merely the entry point; from there, trafficking victims are traded and sold all over the continent.

I spent the days after disembarkation travelling to migrant camps in Sicily, looking for Blessing. When I eventually found her, she told me that if I wanted to understand how she had been trafficked I needed to start in Benin City, in southern Nigeria. So I went there, found her mother, and traced Blessing’s journey across the desert and the sea, piecing together a story of betrayal, desperation, debt bondage, bandits, hope, and death along the ancient caravan routes of the trans-Saharan slave trade.