On July 10, 1908, in his laboratory at Leiden University, the great Dutch physicist Heike Kamerlingh Onnes (1853-1926) experienced the most glorious moment of his career1. That day, after 25 years of hard work and perseverance, of building up from scratch a cryogenic laboratory and organizing superb technical support to run it, he liquefied helium, opening up an entire new research field of low temperature physics. In a triumphant report to the Royal Dutch Academy of Arts and Sciences (KNAW) this historical fact is documented in great detail2,3. Therefore it is remarkable that reliable details about his serendipitous discovery of superconductivity three years later are hard to come by. Lack of information has led to speculations about the discovery, in particular about the doubtful role played by a sleepy “blue boy”4, and about the possible disappearance of Kamerlingh Onnes’s laboratory notebooks. Enough reason, then, to have a close look at the Kamerlingh Onnes archive, stored at Boerhaave Museum in Leiden, to see whether any new clues could be found about the discovery of superconductivity — that most important consequence of the ability to reach liquid-helium temperatures.