In ordinary ice, molecules share a hydrogen atom; but the hydrogen is positioned asymmetrically, closer to one oxygen atom than the other. Under high enough pressure, however, the potential energy curve should change into one with a single minimum, and the hydrogen atom should then sit mid-way between the oxygens, making a symmetric, atomic crystal, ‘ice X’. A new simulation shows that symmetry occurs even earlier, as the quantum-mechanical nature of the of the H atom makes it sit centrally over a small maximum in the energy curve — so there are two kinds of ice X.