  The Earth is bombarded by ultra-relativistic particles, known as cosmic rays
(CRs). CRs with energies up to a few PeV (=10$^{15}$ eV), the knee in the
particle spectrum, are believed to have a Galactic origin. One or more
factories of PeV CRs, or PeVatrons, must thus be active within our Galaxy. The
direct detection of PeV protons from their sources is not possible since they
are deflected in the Galactic magnetic fields. Hundred TeV $\gamma$-rays from
decaying $\pi^0$, produced when PeV CRs collide with the ambient gas, can
provide the decisive evidence of proton acceleration up to the knee. Here we
report the discovery by the High Altitude Water Cherenkov (HAWC) observatory of
the $\gamma$-ray source, HAWC~J1825-134, whose energy spectrum extends well
beyond 200 TeV without a break or cutoff. The source is found to be coincident
with a giant molecular cloud. The ambient gas density is as high as 700
protons/cm$^3$. While the nature of this extreme accelerator remains unclear,
CRs accelerated to energies of several PeV colliding with the ambient gas
likely produce the observed radiation.
