Still alluring and hard to predict at 100.

Birthdays are always a time for introspection and, given the number of fingers humans generally have, a one hundredth birthday is especially so. Almost exactly a century ago, in April 1911, the laboratory run by Heike Kamerlingh Onnes at the University of Leiden discovered a sudden transition to zero resistance in high-purity mercury on cooling through 4.2 K. At the time, Onnes was basking in the glory of winning the race to liquefy helium (see ref. 1 and references therein) and was using this new phase of helium to study the behaviour of matter cooled to newly accessible, lower temperatures. The reason that Onnes was measuring the temperature-dependent electrical resistivity of mercury was to test the proposed ‘death of conductivity’ or ‘freezing out of carrier motion’ that had been proposed by Lord Kelvin as the fate of electrons when absolute zero was approached1. Mercury, given its high vapour pressure, allowed for purification via distillation. The sudden phase transition to an infinitely conducting state was an unimagined result. This first, surprising, appearance of superconductivity was a herald of things to come. Over the ensuing century superconductivity would continue to pop-up in what were thought to be extremely unexpected places, repeatedly: in organic compounds, in oxides, in magnetic compounds, in an overlooked boride and most recently in a whole family of FeAsand FeSe-based compounds. At this point it is sort of like the old Monty Python’s Flying Circus ‘Spanish Inquisition’ skit2,3, with everybody expecting superconductivity to appear more or less wherever it wants to (shown apocryphally in Fig. 1). The simple fact that researchers continue to be surprised by superconductivity’s appearances is evidence that we still do not have a firm grip on what is needed to give rise to this intriguing state. We just celebrated the fiftieth anniversary of the BCS (from Bardeen, Cooper and Schrieffer) theory, which explains