  We briefly review the research on second sound in ultracold atomic physics,
with emphasis on strongly interacting unitary Fermi gases with infinitely large
$s$-wave scattering length. Second sound is a smoking-gun feature of
superfluidity in any quantum superfluids. The observation and characterization
of second sound in ultracold quantum gases has been a long-standing challenge,
and in recent years there are rapid developments due to the experimental
realization of a uniform box-trap potential. The purpose of this review is to
present a brief historical account of the key research activities on second
sound over the past two decades. We summarize the initial theoretical works
that reveal the characteristics of second sound in a unitary Fermi gas, and
introduce its first observation in a highly elongated harmonic trap. We then
discuss the most recent measurement on second sound attenuation in a uniform
setup, which may open a new era to understand quantum transport near quantum
criticality in the strongly interacting regime. The observation of second sound
in homogeneous weakly interacting Bose condensates in both two and three
dimensions are also briefly introduced.
