The road was not really a road. Its two ruts led into Darfur, to the war in western Sudan, from the unmarked border of Chad. So much of the Sahel was like this—unmapped, invisible, yet a boundary nonetheless. The land stretched away in a monotony of gravel pans and dried grasses so translucent—so brittle—they seemed made of blown glass. The iron horizons never budged. Yet we were crossing boundaries with every passing hour, mostly without seeing them.

After I was arrested and imprisoned in Darfur, an American soldier told me, shaking his head in disgust, “You fly over this place and all you see is miles and miles of nothing.” But that was an outsider’s delusion. Every outcrop and plain was parsed by unseen tangents, lines, ghostly demarcations. They portioned off the claims of tribes, individuals, clans. They bulged and recoiled according to war and season. No-go zones encircled water holes. Certain unseen lines, masars, dictated the migration routes of nomads. There was nothing haphazard about any of this. To cross one line or to venture too far from another might invite retribution, even death. And that was the ultimate line of them all in the Sahel: the one between knowing and ignorance.

The Sahel itself is a line.

The word means “shore” in Arabic, which implies a continental margin, a grand beginning and a final end. Stretching across northern Africa roughly along the 13th parallel, the Sahel divides—or unites, depending on your philosophical bent—the sands of the Sahara and Africa’s tropical forests. It is a belt of semiarid grassland that separates (or joins) Arabs and blacks, Muslims and Christians, nomads and farmers, a landscape of greens and a world of tans. Some 50 million of the world’s poorest, most disempowered, most forgotten people hang fiercely on to life there. And for 34 days in Darfur we joined their ranks.