  Dirac semimetals, the materials featured with discrete linearly crossing
points (called Dirac points) between four bands, are critical states of
topologically distinct phases. Such gapless topological states have been
accomplished by a band-inversion mechanism, in which the Dirac points can be
annihilated pairwise by perturbations without changing the symmetry of the
system. Here, we report an experimental observation of Dirac points that are
enforced completely by the crystal symmetry, using a nonsymmorphic
three-dimensional phononic crystal. Intriguingly, our Dirac phononic crystal
hosts four spiral topological surface states, in which the surface states of
opposite helicities intersect gaplessly along certain momentum lines, as
confirmed by our further surface measurements. The novel Dirac system may
release new opportunities for studying the elusive (pseudo)relativistic
physics, and also offer a unique prototype platform for acoustic applications.
