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The KAGRA gravitational-wave detector in Japan is the only operating detectorhosted in an underground infrastructure. Underground sites promise a greatlyreduced contribution of the environment to detector noise thereby opening thepossibility to extend the observation band to frequencies well below 10 Hz. Forthis reason, the proposed next-generation infrastructure Einstein Telescope inEurope would be realized underground aiming for an observation band thatextends from 3 Hz to several kHz. However, it is known that ambient noise inthe low-frequency band 10 Hz - 20 Hz at current surface sites of the Virgo andLIGO detectors is predominantly produced by the detector infrastructure. It isof utmost importance to avoid spoiling the quality of an underground site withnoisy infrastructure, at least at frequencies where this noise can turn into adetector-sensitivity limitation. In this paper, we characterize the KAGRAunderground site to determine the impact of its infrastructure on environmentalfields. We find that while excess seismic noise is observed, its contributionin the important band below 20 Hz is minor preserving the full potential ofthis site to realize a low-frequency gravitational-wave detector. Moreover, weestimate the Newtonian-noise spectra of surface and underground seismic wavesand of the acoustic field inside the caverns. We find that these will likelyremain a minor contribution to KAGRA's instrument noise in the foreseeablefuture.

The KAGRA underground environment and lessons for the Einstein Telescope