  Observation of topological phases beyond two-dimension (2D) has been an open
challenge for ultracold atoms. Here, we realize for the first time a 3D
spin-orbit coupled nodal-line semimetal in an optical lattice and observe the
bulk line nodes with ultracold fermions. The realized topological semimetal
exhibits an emergent magnetic group symmetry. This allows to detect the nodal
lines by effectively reconstructing the 3D topological band from a series of
measurements of integrated spin textures, which precisely render spin textures
on the parameter-tuned magnetic-group-symmetric planes. The detection technique
can be generally applied to explore 3D topological states of similar
symmetries. Furthermore, we observe the band inversion lines from topological
quench dynamics, which are bulk counterparts of Fermi arc states and connect
the Dirac points, reconfirming the realized topological band. Our results
demonstrate the first approach to effectively observe 3D band topology, and
open the way to probe exotic topological physics for ultracold atoms in high
dimensions.
