The Discovery of Superfluidity
Two of the greatest physics discoveries in the first half of the 20th century were superconductivity and superfluidity. Superconductivity involves the frictionless conduction of electricity in wires, and superfluidity the frictionless flow of superfluid liquid helium through channels. Both discoveries led to the arcane phenomenon of macroscopic quantization: the manifestation of the laws of quantum mechanics not just on atomic dimensions but also on the scale of common laboratory apparatus. Both discoveries required the liquefaction of helium, which was first done in 1908.