Underground laboratories provide the ultra-low background and low-vibration environments essential for rare-event searches, gravitational-wave detection, and quantum-sensing technologies. We report a comprehensive environmental characterisation of the Bedretto tunnel in Ticino, Switzerland, a site offering horizontal access, excellent infrastructure, and the potential to be Europe’s second-deepest and quietest underground laboratory. At the prospective physics site, located beneath a granite overburden exceeding 1400 m, we measure fluxes of: cosmic-muon $$(2.54~\pm ~0.97)~\times 10^{-8}~\upmu /\textrm{s} \,\, /\textrm{cm}^2$$ ( 2.54 ± 0.97 ) × 10 - 8 μ / s / cm 2 , $$\gamma $$ γ -ray $$5.67\pm 0.37\, \gamma /\text {cm}^2/$$ 5.67 ± 0.37 γ / cm 2 / $$\text {sec}$$ sec , and neutron fluxes $$(5.56 \pm 0.26)\times 10^{-5}~\textrm{neutrons}/\textrm{cm}^2/$$ ( 5.56 ± 0.26 ) × 10 - 5 neutrons / cm 2 / $$\textrm{sec}$$ sec , as well as the radon concentration, magnetic-field power spectrum, and seismic backgrounds. The muon flux is suppressed by six orders of magnitude relative to the surface, consistent with an effective depth of about 4000 m water equivalent. Gamma-ray and neutron measurements reflect the local geology and guide shielding requirements for future particle and nuclear physics experiments. Magnetic and seismic noise levels are found to be exceptionally low, meeting or exceeding the criteria for next-generation atom-interferometric gravitational-wave detectors. These results establish the site as a highly competitive, accessible deep-underground location for fundamental-physics experiments.