  The simple interpretation of PAO's UHECRs' penetration depth measurements
suggests a transition at the energy range $1.1 - 35 \cdot 10^{18} $ eV from
protons to heavier nuclei. A detailed comparison of this data with air shower
simulations reveals strong restrictions on the amount of light nuclei (protons
and He) in the observed flux. We find a robust upper bound on the observed
proton fraction of the UHECRs flux and we rule out a composition dominated by
protons and He. Acceleration and propagation effects lead to an observed
composition that is different from the one at the source. Using a simple toy
model that take into account these effects, we show that the observations
requires an extreme metallicity at the sources with metals to protons mass
ratio of 1:1, a ratio that is larger by a factor of a hundred than the solar
abundance. This composition imposes an almost impossible constraint on all
current astrophysical models for UHECRs accelerators. This may provide a first
hint towards new physics that emerges at $\sim 100$ TeV and leads to a larger
proton cross section at these energies.
