  Since galaxy clusters sit at the high-end of the mass function, the number of
galaxy clusters both massive and concentrated enough to yield particularly
large Einstein radii poses useful constraints on cosmological and structure
formation models. To date, less than a handful of clusters are known to have
Einstein radii exceeding $\sim40$" (for a source at $z_{s}\simeq2$, nominally).
Here, we report an addition to that list of the Sunyaev-Zel'dovich (SZ)
selected cluster, PLCK G287.0+32.9 ($z=0.38$), the second-highest SZ-mass
($M_{500}$) cluster from the Planck catalog. We present the first strong
lensing analysis of the cluster, identifying 20 sets of multiply-imaged
galaxies and candidates in new \emph{Hubble Space Telescope} data, including a
long, $l\sim22$" giant arc, as well as a quadruply-imaged, apparently bright
(magnified to J$_{F110W}=$25.3 AB), likely high-redshift dropout galaxy at
$z_{phot}=6.90$ [6.13--8.43] (95\% C.I.). Our analysis reveals a very large
critical area (1.55 arcmin$^{2}$, $z_{s}\simeq2$), corresponding to an
effective Einstein radius of $\theta_{E}\sim42$". The model suggests the
critical area will expand to 2.58 arcmin$^{2}$ ($\theta_{E}\sim54$") for
sources at $z_{s}\sim10$. Our work adds to recent efforts to model very massive
clusters towards the launch of the James Webb Space Telescope, in order to
identify the most useful cosmic lenses for studying the early Universe.
Spectroscopic redshifts for the multiply-imaged galaxies and additional HST
data will be necessary for refining the lens model and verifying the nature of
the $z\sim7$ dropout.
