Ice exhibits many solid-state transformations under pressure, and also disp
lays a variety of metastable phases(1). Most of the high-pressure phases of
ice can be recovered at ambient pressure provided that they are first cool
ed below about 100 K. These ice polymorphs might exist on the surfaces of s
everal satellites of the outer planets(2), One of the few exceptions to thi
s (meta) stability on quenching has been ice VII, the dominant high-pressur
e phase. Here we show that isothermal compression of D2O ice VI below 95 K
produces pure ice VII, and that this phase can remain stable at atmospheric
pressure. It remains metastable indefinitely at 77 K. Like the other recov
erable ice phases, it transforms to low-density amorphous ice between about
120 and 150 K at 1 bar. The temperature range over which ice VII remains m
etastable increases markedly on compression to 6 GPa, indicating that ice V
II is in fact the most robust of all the metastable ice phases.