  Higher-order topological insulators and semimetals, which generalize the
conventional bulk-boundary correspondence, have attracted extensive research
interest. Among them, higher-order Weyl semimetals feature two-fold linear
crossing points in three-dimensional (3D) momentum space, 2D Fermi-arc surface
states, and 1D hinge states. Higher-order nodal-point semimetals possessing
Weyl points or Dirac points have been implemented. However, higher-order
nodal-line or nodal-surface semimetals remain to be further explored in
experiments in spite of many previous theoretical efforts. In this work, we
realize a second-order nodal-line semimetal in 3D phononic crystals. The bulk
nodal lines, 2D drumhead surface states guaranteed by Zak phases, and 1D flat
hinge states attributed to kz-dependent quadrupole moments, are observed in
simulations and experiments. Our findings of nondispersive surface and hinge
states may promote applications in acoustic sensing and energy harvesting.
