6 of Zanzibar's best islands: Luxury retreats and underwater adventures: CNN

Mnemba Island: Dubbed the "Millionaire's Island," &Beyond Mnemba Island is wrapped by an oval reef (often referred to as the Mnemba Atoll), which has been declared a marine conservation area. 

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Zanzibar: Located 35 kilometers from Tanzania's mainland, the largest island in an archipelago of dozens is 85 kilometers at its greatest length and 39 kilometers wide.