  What is the highest energy at which gravitons can be observed? We address
this question by studying graviton-to-photon conversion - the
inverse-Gertsenshtein effect - in the magnetic field of the Milky Way. We find
that above $\sim 1~\mbox{PeV}$ the effective photon mass grows large enough to
quench the conversion rate. For sub-PeV energies, the induced photon flux is
comparable to the sensitivity of LHAASO to a diffuse $\gamma$-ray background,
but only for graviton abundances of order $\Omega_{\text{gw}} h^2_0 \sim 1$. 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 of $\Omega_{\text{gw}} h^2_0 \sim 0.01$ could be
detected. We show how 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.
