  Located in a dense complex environment, W51C provides an excellent scenario
to probe accelerated protons in SNRs and their interaction with surrounding
target material. Here we report the observation of extended Very High Energy
(VHE) gamma-ray emission from the W51C supernova remnant (SNR) with MAGIC.
Detections of extended gamma-ray emission in the same region have already been
reported by the Fermi and H.E.S.S. collaborations. Fermi/LAT measured the
source spectrum in the energy range between 0.2 and 50 GeV, which was found to
be well fit by a hadronic neutral-pion decay model. The VHE observations
presented here, obtained with the improved MAGIC stereo system, allow us to
pinpoint the VHE gamma-ray emission in the dense shocked molecular cloud
surrounding the remnant shell. The MAGIC data also allow us to measure, for the
first time, the VHE emission spectrum of W51C from the highest Fermi/LAT
energies up to several TeV. The spatial distribution and spectral properties of
the VHE emission suggest a hadronic origin of the observed gamma rays.
Therefore W51C is a prime candidate for a cosmic ray accelerator.
