  One striking anomaly of water ice has been largely neglected and never
explained. Replacing hydrogen ($^1$H) by deuterium ($^2$H) causes ice to
expand, whereas the "normal" isotope effect is volume contraction with
increased mass. Furthermore, the anomaly increases with temperature $T$, even
though a normal isotope shift should decrease with $T$ and vanish when $T$ is
high enough to use classical nuclear motions. In this study, we show that these
effects are very well described by {\it ab initio} density functional theory.
Our theoretical modeling explains these anomalies, and allows us to predict and
to experimentally confirm a counter effect, namely that replacement of $^{16}$O
by $^{18}$O causes a normal lattice contraction.
