  We report, for the first time, the long-awaited detection of diffuse gamma
rays with energies between 100 TeV and 1 PeV in the Galactic disk.
Particularly, all gamma rays above 398 TeV are observed apart from known TeV
gamma-ray sources and compatible with expectations from the hadronic emission
scenario in which gamma rays originate from the decay of $\pi^0$'s produced
through the interaction of protons with the interstellar medium in the Galaxy.
This is strong evidence that cosmic rays are accelerated beyond PeV energies in
our Galaxy and spread over the Galactic disk.
