Dziani Dzaha is an opaque green crater lake on Petite-Terre, the smaller island of Mayotte in the French Indian Ocean, a few hundred metres from the sea and half again as salty, without oxygen in its depths and holding nothing but microbes, and it is circled on a rim path in forty-five minutes for the sight of green water against the blue of the lagoon beyond. The single organism colouring the lake is Arthrospira, the spiral cyanobacterium that farms harvest and sell as spirulina, blooming so thick that the water is the green of paint, and the shoreline carries stromatolites, layered rock built by microbes that is a fossil almost everywhere on Earth and is still being made here; the crater is a maar blown out by a steam explosion a few thousand years ago, and the rim loop has several steep pitches, none shaded, in a bowl that holds the heat, so it is a morning walk, when the dry brush ringing the crater shelters the island's endemic chameleons and geckos in the cooler hours after sunrise. A short spur drops off the rim to the shore, and up close the lake is nothing like the flat green of the photographs, warm, thin and faintly sulphurous, with volcanic gas bubbling up through the shallows.
Mahoran tradition says a king set out to sacrifice his daughter to the sea, lost his sight on the way and threw her into the crater instead, and the frogs that call there are her voice. Evaporation drops the lake to about three and a half metres by the end of the dry season and drives the surface salinity toward 63, when the green sits at its densest and the stromatolites stand clear of the water, and the scientists who study it treat it as a model of the oceans before oxygen. The lake lies a few minutes from the airport and the ferry to the main island, above the beaches of Moya. Dziani Dzaha is a bowl of green in a volcano, and the only life in it is the kind that came first.