  We present maps of the cosmic large-scale structure around the twelve most
distant galaxy clusters from the Massive Cluster Survey (MACS) as traced by the
projected surface density of galaxies on the cluster red sequence. Taken with
the Suprime-Cam wide-field camera on the Subaru telescope, the images used in
this study cover a 27x27 arcmin^2 area around each cluster, corresponding to 10
x 10 Mpc^2 at the median redshift of z = 0.55 of our sample. We directly detect
satellite clusters and filaments extending over the full size of our imaging
data in the majority of the clusters studied, supporting the picture of mass
accretion via infall along filaments suggested by numerical simulations of the
growth of clusters and the evolution of large-scale structure. A comparison of
the galaxy distribution near the cluster cores with the X-ray surface
brightness as observed with Chandra reveals, in several cases, significant
offsets between the gas and galaxy distribution, indicative of ongoing merger
events. The respective systems are ideally suited for studies of the dynamical
properties of gas, galaxies, and dark matter. In addition, the large-scale
filaments viewed at high contrast in these MACS clusters are prime targets for
the direct detection and study of the warm-hot intergalactic medium (WHIM).
