Graham P. Collins in Scientific American:
Solids and liquids could hardly seem more different, one maintaining a rigid shape and the other flowing to fit the contours of whatever contains it. And of all the things that slosh and pour, superfluids seem to capture the quintessence of the liquid state–running through tiny channels with no resistance and even dribbling uphill to escape from a bowl.
A superfluid solid sounds like an oxymoron, but it is precisely what researchers at Pennsylvania State University have recently witnessed. Physicists Moses Chan and Eun-Seong Kim saw the behavior in helium 4 that was compressed into solidity and chilled to near absolute zero. Although the supersolid behavior had been suggested as a theoretical possibility as long ago as 1969, its demonstration poses deep mysteries.