Helium is one of the most remarkable of all the elements. In its liquid form it continues to surprise and baffle scientists, even 75 years after Heike Kamerlingh Onnes first liquefied helium gas in his laboratory at Leiden University. The Dutch physicist's achievement ended the race to liquefy the most obstinate of gases, but it opened up an intriguing field of research: the study of liquid helium itself. Recently, experiments in Canada have confirmed at last some ideas about the liquid state of helium-4, the commonest form of helium, 45 years after the ideas were first suggested. But this most unusual of liquids still awaits a complete theory of its behaviour.