Maalum cave III (Tanzania, Zanzibar)

These freshwater caves in Zanzibar were formed in a fossil coral rock called reef calcarenite , which was formed from marine skeletons clumped together over millennia. Their natural porosity allows rainwater to slowly seep in, creating fresh water tables beneath the saline soil.

A rare phenomenon on coral islands, these underground reserves have allowed local populations to survive without drawing on seawater or relying entirely on rainwater. Some of these pockets are estimated to date back more than 8,000 years , trapping exceptionally pure water.

The visible diffraction in the water is amplified by the high calcium carbonate content and the extreme transparency of the pool, which allows up to 95% of incident light to pass through—a phenomenon that contributes to these almost supernatural reflections. Biodiversity is low, but the physicochemical conditions of the water attract certain endemic microorganisms, notably thermophilic cyanobacteria.