Oto Cosmo Observatory

Oto Cosmo Observatory is in the center of the Oto-Tentsuji tunnel which is located 100km south of Osaka.

The tunnel which is originally constructed for the railway is 5km long, and its maximum depth is about 470m. Because of the natural ventillation due to the relatively strong wind inside the tunnel, the radon concentration is two or three orders of magnitude lower than the Kamioka underground laboratory. cosmic ray 4e-7 /cm^2/s neutron flux 4e-5 /cm^2/s radon concentration 10 Bq/m^3 Oto Cosmo Obs.(RCNP)