KAGRA underground environment and lessons for the Einstein Telescope

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