  Non-Hermitian topological phases, which exhibit unique features such as skin
effect and exceptional points originated from nontrivial band topologies in
complex plane, have attracted enormous attention in condensed-matter physics
and metamaterials. Here we report the realization of an exceptional line
semimetal in a three-dimensional non-Hermitian phononic crystal. A pair of
exceptional rings with opposite topologies are connected by the drumhead bulk
states in the first Brillouin zone. The exceptional rings not only possess
wave-function topology and thus result in the drumhead surface states, but also
host spectral topology and thereby give rise to the hybrid-order
geometry-dependent skin effect in three dimensions. Our experimental results
evidence the complete non-Hermitian bulk-boundary correspondence of the
three-dimensional exceptional line semimetal, and may pave the way for
designing non-Hermitian acoustic devices.
