  We aim to characterise the properties of a third massive, red supergiant
dominated galactic cluster. To accomplish this we utilised a combination of
near/mid-IR photometry and spectroscopy to identify and classify the properties
of cluster members, and statistical arguments to determine the mass of the
cluster. We found a total of 16 strong candidates for cluster membership, for
which formal classification of a subset yields spectral types from K3-M4 Ia and
luminosities between log(L/L_sun)~4.5-4.8 for an adopted distance of 6+/-1 kpc.
For an age in the range of 16-20 Myr, the implied mass is 2-4x10^4 M_sun,
making it one of the most massive young clusters in the Galaxy. This discovery
supports the hypothesis that a significant burst of star formation occurred at
the base of Scutum-Crux arm between 10-20 Myr ago, yielding a stellar complex
comprising at least ~10^5M_sun of stars (noting that since the cluster
identification criteria rely on the presence of RSGs, we suspect that the true
stellar yield will be significantly higher). We highlight the apparent absence
of X-ray binaries within the star formation complex and finally, given the
physical association of at least two pulsars with this region, discuss the
implications of this finding for stellar evolution and the production and
properties of neutron stars.
