Dziani Dzaha, Crater lake in Dzaoudzi, Mayotte.

Dziani Dzaha is a crater lake located on Petite Terre island at about 6 meters above sea level. The water is surrounded by steep cliffs that form a natural rim and separate the lake from the rest of the island.

Dziani Dzaha occupies a maar, a crater left by the last eruption of a now-extinct volcano between 4,000 and 7,000 years ago; an active magma chamber still lies beneath Petite-Terre. Unlike the basaltic rest of the island, much of the crater walls consist of pale volcanic ash laid down in horizontal layers. It is the most recent and best-preserved volcanic structure in Mayotte.

In Mahorais, dziani means "lake" and dzaha "crater", so the name Dziani Dzaha is literally "crater lake"; the forms lac Dzaha and lac Dziani are also used. The crater walls are mostly pale volcanic ash laid down in horizontal layers, unlike the basalt of the rest of the island.

Visiting requires preparation for moderate hiking conditions around the lake. Weekday visits mean fewer people are present, making for a more relaxed experience.

Below 1.5 metres the lake holds no oxygen at all, so no multicellular life survives there; only at the surface do filamentous cyanobacteria of the genus Arthrospira and alphaproteobacteria forming stromatolites develop. About ten laboratories study these waters, and fourteen genomes of the archaea group Woesearchaeota have been reconstructed from samples.