  Topological nodal-line semimetals are characterized by the line-contact bulk
band crossings and the topological surface states. Breaking certain protecting
symmetry turns this system into a Dirac semimetal or Weyl semimetal that hosts
zero-dimensional isolated nodal points. Recent advances in band theory
predicted a topological nodal-line semimetal state possessing a new type of
nodal line in AlB$_2$-type diborides. Here, we report an experimental
realization of nodal-line fermions and associated surface states near the Fermi
energy in ZrB$_2$ by angle-resolved photoemission spectroscopy combined with
first-principles calculations. The Dirac nodal lines in ZrB$_2$ wind into two
groups of nodal rings, which are linked together along the $\Gamma$-$K$
direction. We further observe a distinct surface state connecting to each nodal
line, indicative of the nontrivial topological nature of the bulk nodal lines.
Therefore, our results provide convincing experimental evidence of the
nodal-line semimetal states in ZrB$_2$ both in the bulk and on the surface,
suggesting ZrB$_2$ as a remarkable platform for discovering unique phenomena
induced by nodal-line fermions.
