  We report the discovery of an extraordinarily massive young cluster of stars
in the Galaxy, having an inferred total initial cluster mass comparable to the
most massive young clusters in the Galaxy. Using {\it IRMOS}, {\it 2MASS}, and
{\it Spitzer} observations, we conclude that there are 14 red supergiants in
the cluster, compared with five, in what was previously thought to be the
richest Galactic cluster of such stars. We infer spectral types from
near-infrared spectra that reveal deep CO bandhead absorption that can only be
fit by red supergiants. We identify a gap of $\Delta${\it K}$_s$$\sim$4
magnitudes between the stars and the bulk of the other stars in the region that
can only be fit by models if the brightest stars in the cluster are red
supergiants. We estimate a distance of 5.8~\kpc to the cluster by associating
an OH maser with the envelope of one of the stars. We also identify a
``yellow'' supergiant of G6~I type in the cluster. Assuming a Salpeter IMF, we
infer an initial cluster mass of 20,000 to 40,000~\Msun for cluster ages of
7-12~\Myr. Continuing with these assumptions, we find 80% of the intial mass
and 99% of the number of stars remain at the present time. We associate the
cluster with an x-ray source (detected by {\it ASCA} and {\it Einstein}), a
recently discovered very high energy $\gamma$-ray source (detected by {\it
INTEGRAL} and {\it HESS}), and several non-thermal radio sources, finding that
these objects are likely related to recent supernovae in the cluster. In
particular, we claim that the cluster has produced at least one recent
supernova remnant with properties similar to the Crab nebula. It is not
unlikely to find such a source in this cluster, given our estimated supernova
rate of one per 40,000 to 80,000~{\it yr}.
