  Diffuse gamma-ray emission has long been established as the most prominent
feature in the GeV sky. Although the imaging atmospheric Cherenkov technique
has been successful in revealing a large population of discrete TeV gamma-ray
sources, a thorough investigation of diffuse emission at TeV energies is still
pending. Data from the Galactic Plane Survey (GPS) obtained by the High Energy
Stereoscopic System (H.E.S.S.) have now achieved a sensitivity and coverage
adequate for probing signatures of diffuse emission in the energy range of ~100
GeV to a few TeV. Gamma-rays are produced in cosmic-ray interactions with the
interstellar medium (aka "sea of cosmic rays") and in inverse Compton
scattering on cosmic photon fields. This inevitably leads to guaranteed
gamma-ray emission related to the gas content along the line-of-sight. Further
contributions relate to those gamma-ray sources that fall below the current
detection threshold and the aforementioned inverse Compton emission. Based on
the H.E.S.S. GPS, we present the first observational assessment of diffuse TeV
gamma-ray emission. The observation is compared with corresponding flux
predictions based on the HI (LAB data) and CO (as a tracer of H2, NANTEN data)
gas distributions. Consequences for unresolved source contributions and the
anticipated level of inverse Compton emission are discussed.
