Discovering the most elusive radio relic in the sky: Diffuse Shock
  Acceleration caught in the act?
  The origin of radio relics is usually explained via diffusive shock
acceleration (DSA) or re-acceleration of electrons at/from merger shocks in
galaxy clusters. The case of acceleration is challenged by the low predicted
efficiency of low-Mach number merger shocks, unable to explain the power
observed in most radio relics. In this Letter we present the discovery of a new
giant radio relic around the galaxy cluster Abell 2249 ($z=0.0838$) using
LOFAR. It is special since it has the lowest surface brightness of all known
radio relics. We study its radio and X-ray properties combinig LOFAR data with
uGMRT, JVLA and XMM. This object has a total power of $L_{1.4\rm GHz}=4.1\pm
0.8 \times 10^{23}$ W Hz$^{-1}$ and integrated spectral index $\alpha = 1.15\pm
0.23$. We infer for this radio relic a lower bound on the magnetisation of
$B\geq 0.4\, \mu$G, a shock Mach number of $\mathcal{M}\approx 3.79$, and a low
acceleration efficiency consistent with DSA. This result suggests that a
missing population of relics may become visible thanks to the unprecedented
sensitivity of the new generation of radio telescopes.
