Dziani Dzaha is an opaque green crater lake on Petite-Terre, the smaller island of Mayotte, a hundred metres from the Indian Ocean. The green is one organism. Arthrospira, the spiral cyanobacterium that farms harvest and sell as spirulina, blooms here without stopping and accounts for more than nine-tenths of everything photosynthesising in the water. In the soda lakes of East Africa the same genus feeds flamingos. Here nothing eats it, because there is nothing in the lake to do the eating. The water is half again as salty as the sea, alkaline, and without oxygen below chest depth, and nothing with more than one cell lives in it. The bowl was blown open by steam a few thousand years ago, where rising magma met groundwater.
The rim loop takes about forty-five minutes; several pitches are steep, none of them are shaded, and the crater holds its heat around you, so the walk belongs to the morning. The gaps in the palm and scrub keep framing the two waters against each other. At the seaward high point you get both: flat green on one side, open lagoon on the other, a hundred metres apart. The dry brush along the way holds Mayotte's own chameleons, which freeze rather than run, so you can pass within a metre of one and never see it. A short spur takes you off the rim down to the shore. Up close the lake is nothing like the flat green of the photographs: warm, thin, faintly sulphurous, with volcanic gas coming up through the shallows. The lumpy grey mounds along the waterline are the reason scientists come. They are stromatolites, rock laid down film by film by microbial mats, and almost everywhere else on Earth they survive only as fossils. Structures like these left the oldest evidence of life on the planet, more than three billion years ago, and these ones are still being built.