While several experiments yielded negative results, in the 1980s, John Goodkind discovered the first anomaly in a solid by using ultrasound. Inspired by his observation, in 2004 Eun-Seong Kim and Moses Chan at Pennsylvania State University saw phenomena which were interpreted as supersolid behavior. Specifically, they observed a non-classical rotational moment of inertia of a torsional oscillator. This observation could not be explained by classical models but was consistent with superfluid-like behavior of a small percentage of the helium atoms contained within the oscillator.
This observation triggered a large number of follow-up studies to reveal the role played by crystal defects or helium-3 impurities. Further experimentation has cast some doubt on the existence of a true supersolid in helium. Most importantly, it was shown that the observed phenomena could be largely explained due to changes in the elastic properties of the helium. In 2012, Chan repeated his original experiments with a new apparatus that was designed to eliminate any such contributions. In this experiment, Chan and his coauthors found no evidence of supersolidity.