  The discovery of the quantum Hall effect in 2D systems opens the door to
topological phases of matter. A quantum Hall effect in 3D is a long-sought
phase of matter and has inspired many efforts and claims. In the perspective,
we review our proposal that guarantees a 3D quantum Hall effect. The proposal
employs the topologically-protected Fermi arcs and the "wormhole" tunneling via
the Weyl nodes in a 3D topological semimetal. The 1D edge states in this 3D
quantum Hall effect show an example of (d-2)-dimensional boundary states.
Possible signatures of the 3D quantum Hall effect have been observed in the
topological Dirac semimetals, but with many questions, which will attract more
works to verify the mechanism and realize the 3D quantum Hall in the future.
