You may have heard of superfluids and superconductors, so why not supersolids? In 2004 Moses Chan and Eunseong Kim thought they had discovered that super-cooled helium ice could essentially walk through walls – a defining characteristic of a supersolid.
The experiment was to make a cylinder with tiny nanopores in its walls, fill the pores with solid helium ice, suspend the cylinder from a torsional spring, and then give it a little twist. Like a kid on a swing set, the cylinder started rotating back and forth, with a frequency depending on its mass. As they supercooled the cylinder even further, they saw that the oscillation frequency changed, as if it had less mass!