  Two-dimensional topological edge states, immunizing against defects and
disorders, have greatly revolutionized our scientific cognition on propagation
and scattering of acoustic waves. Recently, the similar states have been
predicted in three-dimensional acoustic systems consisting of chiral coupling
resonance cavities. However, the direct observation of topological surface
propagation, especially in a 3D acoustic system with simple scatterers but not
complicatedly coupling resonances, has still not been exploited. Here, we
design and fabricate a 3D acoustic semimetal composed of rotationally stacked
rods. This semimetal not only produces a linear Weyl degeneracy with a charge
of 1, but also yields a double Weyl point with a charge of 2. In its nontrivial
gaps, we discover the surface states associated with these Weyl points and
experimentally demonstrate the topologically protected one-way propagation of
acoustic waves. Due to its good fabricability and topological non-reciprocity,
this acoustic semimetal provides an excellent platform for the integration of
various acoustic devices, from a macroscopic scale to a nanoscale.
