  We present a wide-field (30' diameter) 850um SCUBA-2 map of the spectacular
three-component merging supercluster, RCS 231953+00, at z=0.9. The brightest
submillimetre galaxy (SMG) in the field (S_850=12mJy) is within 30" of one of
the cluster cores (RCS 2319-C), and is likely to be a more distant, lensed
galaxy. Interestingly, the wider field around RCS 2319-C reveals a local
overdensity of SMGs, exceeding the average source density by a factor of 4.5,
with a <1 per cent chance of being found in a random field. Utilizing
Herschel-SPIRE observations, we find three of these SMGs have similar
submillimetre colours. We fit their observed 250-850um spectral energy
distributions to estimate their redshift, yielding 2.5<z<3.5, and calculate
prodigious star formation rates (SFRs) ranging from 500-2500 solar masses per
year. We speculate that these galaxies are either lensed SMGs, or signpost a
physical structure at z~3: a 'protocluster' inhabited by young galaxies in a
rapid phase of growth, destined to form the core of a massive galaxy cluster by
z=0.
