Off the northeast tip of Zanzibar lies Mnemba Atoll, a tiny private island surrounded by a necklace of blindingly white sandbars that partially vanish beneath the Indian Ocean when the tide comes in. The warm, shallow water around it is so clear and calm it's basically a giant natural swimming pool teeming with sea turtles and tropical fish. This is the kind of place where people book a two-week vacation and suddenly realize they never want to go home.