Vikings have long been acknowledged as fierce warriors and explorers, but their sorceresses are lesser known: They held a key role in Norse society and are now the subject of a Danish exhibit. The half-sorceress half-druid known as a "volva" is "a kind of female shaman, a seeress, a spywoman," says Peter Pentz, a researcher who helped curate the National Museum of Denmark's exhibit. According to the Viking sagas -- based on oral accounts and written several hundred years after the 9th-11th century Viking era -- volvas were called upon to predict the future and could summon the gods. IMAGES AND SOUNDBITES