  Lorentz-violating type-II Dirac nodal line semimetals (DNLSs), hosting curves
of band degeneracy formed by two dispersion branches with the same sign of
slope, represent a novel states of matter. While being studied extensively in
theory, convincing experimental evidences of type-II DNLSs remain elusive.
Recently, Vanadium-based kagome materials have emerged as a fertile ground to
study the interplay between lattice symmetry and band topology. In this work,
we study the low-energy band structure of double-kagome-layered
CsV$_8$Sb$_{12}$ and identify it as a scarce type-II DNLS protected by mirror
symmetry. We have observed multiple DNLs consisting of type-II Dirac cones
close to or almost at the Fermi level via angle-resolved photoemission
spectroscopy (ARPES). First-principle analyses show that spin-orbit coupling
only opens a small gap, resulting effectively gapless ARPES spectra, yet
generating large spin Berry curvature. These type-II DNLs, together with the
interaction between a low-energy van Hove singularity and quasi-1D band as we
observed in the same material, suggest CsV$_8$Sb$_{12}$ as an ideal platform
for exploring novel transport properties such as chiral anomaly, the Klein
tunneling and fractional quantum Hall effect.
