  We present the discovery of an exceptional dimming event in a cool supergiant
star in the Local Volume spiral M51. The star, dubbed M51-DS1, was found as
part of a Hubble Space Telescope (HST) search for failed supernovae (SNe). The
supergiant, which is plausibly associated with a very young ($\lesssim6$ Myr)
stellar population, showed clear variability (amplitude $\Delta
F814W\approx0.7$ mag) in numerous HST images obtained between 1995 and 2016,
before suddenly dimming by $>$2 mag in $F814W$ sometime between late 2017 and
mid-2019. In follow-up data from 2021, the star rebrightened, ruling out a
failed supernova. Prior to its near-disappearance, the star was luminous and
red ($M_{F814W}\lesssim-7.6$ mag, $F606W-F814W=1.9$ - $2.2$ mag). Modeling of
the pre-dimming spectral energy distribution of the star favors a highly
reddened, very luminous ($\log[L/L_{\odot}] = 5.4$ - $5.7$) star with
$T_{\mathrm{eff}}\approx3700$ - $4700$ K, indicative of a cool yellow or
post-red supergiant (RSG) with an initial mass of $\approx26$ - $40$
$M_{\odot}$. However, the local interstellar extinction and circumstellar
extinction are uncertain, and could be lower: the near-IR colors are consistent
with an RSG, which would be cooler ($T_{\mathrm{eff}}\lesssim3700$ K) and
slightly less luminous ($\log[L/L_{\odot}] = 5.2$ - $5.3$), giving an inferred
initial mass of $\approx19$ - $22$ $M_{\odot}$. In either case, the dimming may
be explained by a rare episode of enhanced mass loss that temporarily obscures
the star, potentially a more extreme counterpart to the 2019 - 2020 "Great
Dimming" of Betelgeuse. Given the emerging evidence that massive evolved stars
commonly exhibit variability that can mimic a disappearing star, our work
highlights a substantial challenge in identifying true failed SNe.
