Entertaining voyages into the geography of the imagination, from a sailor and journalist (Charting the Sea of Darkness, not reviewed). In the early days of cartography, islands came and islands went. Unable to plot longitude, explorers should have advised their mapmakers: Here there be islands, maybe. There were other reasons for these illusory shards of terra firma: mirages and delusions, shape-shifting mountains that emerged and then sank again beneath the waves, and offshore banks. In the North Atlantic, a few of these phantom isles have persisted in our imaginations--it is their story that Johnson relates. There is the Isle of Demons, said to be populated by beasts and evil spirits. Johnson fancies the demonic cries heard by mariners were those of nesting pelagic birds, perhaps on Funk Island. There is the curious disappearance of the inhabited island of Frisland, supposedly discovered by the 14th-century voyager Nicolo Zeno. Buss Island, frequently sighted by ships looking for the Northwest Passage, is now gone; was it just another ""false horizon created by the tricks and deceptive appearances of the Arctic atmosphere""? Consider Antilla, its seven cities home to seven refugee bishops, and Hy-Brazil, revealed once every seven years when its veil of fog lifts. Easily one of the best tales is that of Saint Ursula and her 11,000 virgin companions, martyred by Huns at the behest of the Roman emperor. The islands named after her are no phantoms--they sit clear as day in the Caribbean, tagged by Christoper Columbus--but her legend may well be, suggests Johnson. He pulls together as much as he can about vanished or fabulous islands, plumbing ancient texts for sightings and commentary, poring over early maps that chart the peregrinations of the islands, then serves up his findings with a light, bright touch. A feast for those who hunger after terra incognita.