Lac Rose is a Senegalese lake salty enough for a single alga to colour it, and shallow enough that three thousand people work it on foot. It lies thirty kilometres north of Dakar, and the salt in it runs as high as forty percent. The pink comes from Dunaliella salina, which floods its cells with red pigment to survive the light in that brine. Whether you see it depends on the month. Evaporation through the dry months drives the salinity up and the water deepens from pale rose to strawberry, strongest from late January into early March. The rains from July dilute it again, the colour dulls and can turn green, and in bad years it has gone altogether. The work does not change.
The collectors stand chest-deep, shea butter smeared over their skin against the brine. They break the crust off the bottom with a pole, fill their baskets by feel, and tip them into pirogues until the boats sit almost level with the water. The loads go ashore on women's heads, onto the white mounds along the dunes. Almost none of it stays in Senegal. Most travels across West Africa to keep fish from spoiling — including the dried fish in thieboudienne, the rice-and-fish plate Senegal treats as its own. The lake itself is a recent arrival. Within living memory this was fresh water with fish in it. Drifting dunes sealed its channel to the ocean, the droughts of the 1970s shrank the rest, and the sea still supplies every grain of the salt, seeping through that barrier. The ocean is right there: you cross the dune strip on foot in minutes, still brine on one side, Atlantic surf on an empty beach on the other. You walk into the lake and the water pushes you straight back up, dense, faintly oily, and unforgiving to a cut. The salt keeps arriving faster than anyone lifts it out. Left alone the basin would fill with its own crystal and close, so the three thousand people standing in it are the reason there is still a lake.