Second sound is an unusual type of propagating wave mode, which can occur in superfluids, involving fluctuations in the local temperature and entropy of the medium rather than in the local density and pressure as found in a conventional sound wave. The phenomenon has been investigated, both experimentally and theoretically, in the common helium isotope, 4He, over many years; but its observation in 3He, whose ultra-low temperature superfluidity is still a relatively recent (1972) discovery, was reported for the first time by Corruccini and Osheroff only last year.