  Standard inflationary hot big bang cosmology predicts small fluctuations in
the Cosmic Microwave Background (CMB) with isotropic Gaussian statistics. All
measurements support the standard theory, except for a few anomalies discovered
in the Wilkinson Microwave Anisotropy Probe maps and confirmed recently by the
Planck satellite. The Cold Spot is one of the most significant of such
anomalies, and the leading explanation of it posits a large void that imprints
this extremely cold area via the linear Integrated Sachs-Wolfe (ISW) effect due
to the decay of gravitational potentials over cosmic time, or via the
Rees-Sciama (RS) effect due to late-time non-linear evolution. Despite several
observational campaigns targeting the Cold Spot region, to date no suitably
large void was found at higher redshifts $z > 0.3$. Here we report the
detection of an $R =(192 \pm 15) h^{-1}Mpc$ size supervoid of depth $\delta =
-0.13 \pm 0.03$, and centred at redshift $z = 0.22$. This supervoid, possibly
the largest ever found, is large enough to significantly affect the CMB via the
non-linear RS effect, as shown in our Lemaitre-Tolman-Bondi framework. This
discovery presents the first plausible explanation for any of the physical CMB
anomalies, and raises the possibility that local large-scale structure could be
responsible for other anomalies as well.
