One of the strangest anomalies thus far exhibited by matter has been the special thermal wave property of liquid helium II, known assecond sound. Characteristic of no other substance than helium II—that weird form adopted by liquid helium below the &lgr;‐temperature of 2.19°K—second sound is essentially an undamped thermal wave propagation. Such thermal waves display all the usual properties of wave phenomena, including resonance and reflection characteristics. This property of heat flow conforming to a wave equation, rather than to the classical diffusive heat flow equation, results in such seemingly paradoxical situations as heat flowing uphill against thermal gradients. Anomalous even in name, second sound never activates microphones and is generated by heat impulses rather than by mechanical impulses. Finally, its behaviour provides perhaps the most effective means for investigating and understanding the true nature of this so‐calledquantum liquid, helium II, and the associated quantum hydrodynamics.