Just offshore of Tanna, the southernmost island in Vanuatu, the tiny atoll known as Mystery Island is your vision of a castaway paradise: an uninhabited ring of white sand beaches set in luminous turquoise waters, with palms and pandanus fringing the shorelines. It is not large, barely two-thirds of a mile long and a little more than a couple of hundred yards wide. The friendly folk from nearby Aneityum island come over to chat and offer handcrafts and trinkets for your consideration. Activities might include stand-up paddle-boarding, snorkeling, swimming or simply strolling some of the trails that bisect the island.