Lake Natron, in the Rift Valley of northern Tanzania, is where most of the lesser flamingos alive were hatched, in water caustic enough to burn skin and to turn drowned birds to stone. It has no outlet: hot springs feed it, the heat boils it down, and what is left is a brine of sodium carbonate stained red by the microbes that can live in it. Almost nothing else can. The exception is the lesser flamingo, built for the poison: scaly skin protects its legs, and a combed bill strains out the alkaline algae that dye both the bird and the water pink. At the end of the dry season the flamingos arrive in their millions and nest on soda islets out in the middle, where nothing that hunts them can cross. You watch the nests from firm ground through binoculars; walking closer puts the whole colony in the air and the eggs are abandoned.
Along the shore the soda has dried into pale tiles that crunch under your boots, the heat stands in a shimmer and the air carries a faint smell of ammonia. The calcified birds along it are real; the famous photographs of them standing frozen in death were posed. From down here the lake reads as a dull sheet. The crimson only shows from above, from the escarpment or a light aircraft, in deep red whorls veined with white. The horizon belongs to Ol Doinyo Lengai, the Maasai's Mountain of God and the only volcano on Earth still erupting carbonatite. That lava is soda rather than rock, so runny it pours like engine oil, black when it lands and crumbling to ghost white within hours. You leave in the dark and grind up hours of loose ash to reach the rim at dawn. Below it a guide takes you up the hidden Ngare Sero gorge, wading a clear stream between palms and volcanic walls to a chain of cool waterfalls, fresh water a few kilometres from water that would burn. The chicks out on those islets will be three in every four lesser flamingos alive.