  Metals are canonical plasmonic media at infrared and optical wavelengths,
allowing one to guide and manipulate light at the nano-scale. A special form of
optical waveguiding is afforded by highly anisotropic crystals revealing the
opposite signs of the dielectric functions along orthogonal directions. These
media are classified as hyperbolic and include crystalline insulators,
semiconductors and artificial metamaterials. Layered anisotropic metals are
also anticipated to support hyperbolic waveguiding. Yet this behavior remains
elusive, primarily because interband losses arrest the propagation of infrared
modes. Here, we report on the observation of propagating hyperbolic waves in a
prototypical layered nodal-line semimetal ZrSiSe. The observed waveguiding
originates from polaritonic hybridization between near-infrared light and
nodal-line plasmons. Unique nodal electronic structures simultaneously suppress
interband loss and boost the plasmonic response, ultimately enabling the
propagation of infrared modes through the bulk of the crystal.
