What is the highest energy at which gravitons can be observed? This question can be addressed by studying graviton-to-photon conversion - the inverse-Gertsenshtein effect - in the magnetic field of the Milky Way. Above ~ 1 PeV the effective photon mass grows large enough to quench the conversion rate. In the sub-PeV range, the induced photon flux is comparable to the sensitivity of LHAASO to a diffuse gamma-ray background, but only for graviton abundances of order of the total energy density in the Universe. In the future, owing to a better understanding of gamma-ray backgrounds, larger effective areas and longer observation times, sub-PeV shimmering gravitons with a realistic abundance ~0.01 of the total energy density could be detected. We show that such a large abundance is achieved in a cosmologically-motivated scenario of post-recombination superheavy dark matter decay. Therefore, the sub-PeV range might be the ultimate energy frontier at which gravitons can be observed.
