Ice XVIII

Super-compressed water seems to form a new state of matter in which the oxygen atoms form a lattice and the protons migrate around within it. That's not quite what you think of as a solid, nor what you think of as a liquid. It was proposed years ago, and appears to be verified by some new experiments. Powerful lasers blast diamond anvils, and the resulting explosion propels them into each other, compressing the bit of water in between them. Another set of lasers vaporizes iron at the side, letting the experimenters do X-ray crystallography.

Maybe Uranus and Neptune have some of this in their cores--or a mix of this and some other stuff. It might explain their weird magnetic fields.

I wonder what other bulk properties you could get?