  In recent years, the outskirts of galaxy clusters have emerged as one of the
new frontiers and unique laboratories for studying the growth of large scale
structure in the universe. Modern cosmological hydrodynamical simulations make
firm and testable predictions of the thermodynamic and chemical evolution of
the X-ray emitting intracluster medium. However, recent X-ray and
Sunyaev-Zeldovich effect observations have revealed enigmatic disagreements
with theoretical predictions, which have motivated deeper investigations of a
plethora of astrophysical processes operating in the virialization region in
the cluster outskirts. Much of the physics of cluster outskirts is
fundamentally different from that of cluster cores, which has been the main
focus of X-ray cluster science over the past several decades. A next-generation
X-ray telescope, equipped with sub-arcsecond spatial resolution over a large
field of view along with a low and stable instrumental background, is required
in order to reveal the full story of the growth of galaxy clusters and the
cosmic web and their applications for cosmology.
