  Emergent phases often appear when the electronic kinetic energy is comparable
to the Coulomb interactions. One approach to seek material systems as hosts of
such emergent phases is to realize localization of electronic wavefunctions due
to the geometric frustration inherent in the crystal structure, resulting in
flat electronic bands. Recently, such efforts have found a wide range of exotic
phases in the two-dimensional kagome lattice, including magnetic order,
time-reversal symmetry breaking charge order, nematicity, and
superconductivity. However, the interlayer coupling of the kagome layers
disrupts the destructive interference needed to completely quench the kinetic
energy. Here we demonstrate that an interwoven kagome network-a pyrochlore
lattice-can host a three dimensional (3D) localization of electron
wavefunctions. Meanwhile, the nonsymmorphic symmetry of the pyrochlore lattice
guarantees all band crossings at the Brillouin zone X point to be 3D gapless
Dirac points, which was predicted theoretically but never yet observed
experimentally. Through a combination of angle-resolved photoemission
spectroscopy, fundamental lattice model and density functional theory
calculations, we investigate the novel electronic structure of a Laves phase
superconductor with a pyrochlore sublattice, CeRu$_2$. We observe flat bands
originating from both the Ce 4$f$ orbitals as well as from the 3D destructive
interference of the Ru 4$d$ orbitals. We further observe the nonsymmorphic
symmetry-protected 3D gapless Dirac cones at the X point. Our work establishes
the pyrochlore structure as a promising lattice platform to realize and tune
novel emergent phases intertwining topology and many-body interactions.
