  We present an IGM HI tomography map in a survey volume of $16 \times 19
\times 131 \ h^{-3} {\rm comoving \ Mpc}^{3}$ (cMpc$^3$) centered at MAMMOTH-1
nebula and three neighbouring quasars at $z=2.3$. MAMMOTH-1 nebula is an
enormous Ly$\alpha$ nebula (ELAN), hosted by a type-II quasar dubbed
MAMMOTH1-QSO, that extends over $1\ h^{-1}$ cMpc with not fully clear physical
origin. Here we investigate the HI-gas distribution around MAMMOTH1-QSO with
the ELAN and three neighbouring type-I quasars, making the IGM HI tomography
map with a spatial resolution of $2.6\ h^{-1}$ cMpc. Our HI tomography map is
reconstructed with HI Ly$\alpha$ forest absorption of bright background objects
at $z=2.4-2.9$: one eBOSS quasar and 16 Keck/LRIS galaxy spectra. We estimate
the radial profile of HI flux overdensity for MAMMOTH1-QSO, and find that
MAMMOTH1-QSO resides in a volume with significantly weak HI absorption. This
suggests that MAMMOTH1-QSO has a proximity zone where quasar illuminates and
photo-ionizes the surrounding HI gas and suppresses HI absorption, and that the
ELAN is probably a photo-ionized cloud embedded in the cosmic web. The HI
radial profile of MAMMOTH1-QSO is very similar to those of three neighbouring
type-I quasars at $z=2.3$, which is compatible with the AGN unification model.
We compare the distributions of the HI absorption and star-forming galaxies in
our survey volume, and identify a spatial offset between density peaks of
star-forming galaxies and HI gas. This segregation may suggest anisotropic UV
background radiation created by star-forming galaxy density fluctuations.
