  Superfluidity in its various forms has fascinated scientists since the
observation of frictionless flow in liquid helium II. In three spatial
dimensions (3D), it is conceptually associated with the emergence of long-range
order (LRO) at a critical temperature $T_{\text{c}}$. One of its hallmarks,
predicted by the highly successful two-fluid model and observed in both liquid
helium and ultracold atomic gases, is the existence of two kinds of sound
excitations, the first and second sound. In 2D systems, thermal fluctuations
preclude LRO, but superfluidity nevertheless emerges at a nonzero
$T_{\text{c}}$ via the infinite-order Berezinskii-Kosterlitz-Thouless (BKT)
transition, which is associated with a universal jump in the superfluid density
$n_{\text{s}}$ without any discontinuities in the fluid's thermodynamic
properties. BKT superfluids are also predicted to support two sounds, but the
observation of this has remained elusive. Here we observe first and second
sound in a homogeneous 2D atomic Bose gas, and from the two
temperature-dependent sound speeds extract its superfluid density. Our results
agree with BKT theory, including the prediction for the universal
superfluid-density jump.
