  We report a $z=2.30$ galaxy protocluster (COSTCO-I) in the COSMOS field,
where the Lyman-$\alpha$ forest as seen in the CLAMATO IGM tomography survey
does not show significant absorption. This departs from the
transmission-density relationship (often dubbed the fluctuating Gunn-Peterson
approximation; FGPA) usually expected to hold at this epoch, which would lead
one to predict strong Ly$\alpha$ absorption at the overdensity. For comparison,
we generate mock Lyman-$\alpha$ forest maps by applying FGPA to constrained
simulations of the COSMOS density field, and create mocks that incorporate the
effects of finite sightline sampling, pixel noise, and Wiener filtering.
Averaged over $r=15\,h^{-1}\,\mathrm{Mpc}$ around the protocluster, the
observed Lyman-$\alpha$ forest is consistently more transparent in the real
data than in the mocks, indicating a rejection of the null hypothesis that the
gas in COSTCO-I follows FGPA ($p=0.0026$, or $2.79 \sigma$ significance). It
suggests that the large-scale gas associated with COSTCO-I is being heated
above the expectations of FGPA, which might be due to either large-scale AGN
jet feedback or early gravitational shock heating. COSTCO-I is the first known
large-scale region of the IGM that is observed to be transitioning from the
optically-thin photoionized regime at Cosmic Noon, to eventually coalesce into
an intra-cluster medium (ICM) by $z=0$. Future observations of similar
structures will shed light on the growth of the ICM and allow constraints on
AGN feedback mechanisms.

Astrophysics of Galaxies