Dziani Dzaha has no fish, no shrimp, no snails — nothing multicellular lives in it at all. The green lake, 660 metres across, fills a maar: a crater blown open by steam when rising magma met groundwater a few thousand years ago, and the youngest ground anywhere on Mayotte. The Indian Ocean is barely a hundred metres away, but the two never meet at the surface. Seawater seeps in through the rock, the tropical sun evaporates it, and the salt stays behind, so what fills the crater is half again as salty as the sea, alkaline above pH 9, thick with dissolved carbonate and free of oxygen below chest depth. One organism thrives in that: Arthrospira, the spiral cyanobacterium sold everywhere else as spirulina. It accounts for more than 97 percent of the lake's photosynthetic life and blooms without pause, which is why the water reads as flat, opaque green rather than blue.
A marked path circles the crater rim in about 45 minutes, through coconut palm and dry scrub, and every gap in the branches frames that green against the blue of the lagoon. At the shore the lake loses its poster-paint look — warm, thin, faintly sulphurous, with volcanic carbon dioxide bubbling up through the shallows and lumpy grey stromatolites growing at the waterline. Swimming and fishing are both banned. The lake itself has been changing since May 2018, when an earthquake swarm and the birth of a submarine volcano 50 kilometres east dropped the whole of Mayotte by 10 to 20 centimetres; fractured rock has since let seawater leak in, and monitoring has recorded a lasting shift in the microbes living here.