  Based on sub-arcsecond Atacama Large Millimeter/submillimeter Array (ALMA)
and Submillimeter Array (SMA) 1.3 mm continuum images of the massive
protocluster NGC 6334I obtained in 2015 and 2008, we find that the dust
emission from MM1 has increased by a factor of 4.0$\pm$0.3 during the
intervening years, and undergone a significant change in morphology. The
continuum emission from the other cluster members (MM2, MM4 and the UCHII
region MM3 = NGC 6334F) has remained constant. Long term single-dish maser
monitoring at HartRAO finds that multiple maser species toward NGC 6334I flared
beginning in early 2015, a few months before our ALMA observation, and some
persist in that state. New ALMA images obtained in 2016 July-August at 1.1 and
0.87 mm confirm the changes with respect to SMA 0.87 mm images from 2008, and
indicate that the (sub)millimeter flaring has continued for at least a year.
The excess continuum emission, centered on the hypercompact HII region MM1B, is
extended and elongated ($1.6" \times 1.0" \approx 2100 \times 1300$ au) with
multiple peaks, suggestive of general heating of the surrounding subcomponents
of MM1, some of which may trace clumps in a fragmented disk rather than
separate protostars. In either case, these remarkable increases in maser and
dust emission provide direct observational evidence of a sudden accretion event
in the growth of a massive protostar yielding a sustained luminosity surge by a
factor of $70\pm20$, analogous to the largest events in simulations by Meyer et
al. (2017). This target provides an excellent opportunity to assess the impact
of such a rare event on a protocluster over many years.
