The early voyages into the deep waters of the Atlantic rank among the greatest feats of exploration. In tiny, apparently fragile vessels the Irish monks searched for desolate places in the ocean in which to pursue their vocation. Their successors, the Vikings, with their superb shipbuilding skills, created fast, seaworthy craft which took them far out into the unknown, until they finally reached Greenland and America. The author of this book discusses the history of those expeditions not only as a scholar and historian, but also as a practical sailor. Besides the problem of what the early explorers actually achieved, he poses the question of how they did it, without compass, quadrant or astrolabe.