  The KAGRA gravitational-wave detector in Japan is the only operating detector
hosted in an underground infrastructure. Underground sites promise a greatly
reduced contribution of the environment to detector noise thereby opening the
possibility to extend the observation band to frequencies well below 10 Hz. For
this reason, the proposed next-generation infrastructure Einstein Telescope in
Europe would be realized underground aiming for an observation band that
extends from 3 Hz to several kHz. However, it is known that ambient noise in
the low-frequency band 10 Hz - 20 Hz at current surface sites of the Virgo and
LIGO detectors is predominantly produced by the detector infrastructure. It is
of utmost importance to avoid spoiling the quality of an underground site with
noisy infrastructure, at least at frequencies where this noise can turn into a
detector-sensitivity limitation. In this paper, we characterize the KAGRA
underground site to determine the impact of its infrastructure on environmental
fields. We find that while excess seismic noise is observed, its contribution
in the important band below 20 Hz is minor preserving the full potential of
this site to realize a low-frequency gravitational-wave detector. Moreover, we
estimate the Newtonian-noise spectra of surface and underground seismic waves
and of the acoustic field inside the caverns. We find that these will likely
remain a minor contribution to KAGRA's instrument noise in the foreseeable
future.
