In a letter to Nature [2004, 427, 225], E. Kim and M. H. W. Chan (Pennsylvania State University, USA) note that when liquid ⁴He is cooled below 2.176 K, it undergoes a phase transition and becomes a superfluid with zero viscosity. They claim that in addition to superflow in the liquid phase, superflow can also occur under some conditions in the solid phase of one of the helium isotopes (⁴He), and present results to back this up. In other words - evidence for a "supersolid". A supersolid behaves like a superfluid (flows without resistance) although it has crystalline solid characteristics.

See Nature for further datails.