mBq m−3, and in water output by the system, 0.4±0.2 mBq m−3. Air purification system Purified air is supplied in the gap between the water surface and the top of the Super-Kamiokande tank. The air purification system contains three compressors, a buffer tank, dryers, filters, and activated charcoal filters. A total of 8 m3 of activated charcoal is used. The last 50 L of charcoal is cooled to −40 °C to increase removal efficiency for radon. Typical flow rates, dew point, and residual radon concentration are 18 m3/h, −65 °C (@+1 kg/cm2), and a few mBq m−3, respectively. Typical radon concentration in the dome air is measured to be 40 Bq m−3. Radon levels in the mine tunnel air, near the tank cavity dome, typically reach 2000–3000 Bq m−3 during the warm season, from May until October, while from November to April the radon level is approximately 100–300 Bq m−3. This variation is due to the chimney effect in the ventilation pattern of the mine tunnel system; in cold seasons, fresh air flows into the Atotsu tunnel entrance that is a relatively short path through exposed rock before reaching the experimental area, while in the summer, air flows out the tunnel, drawing radon-rich air from deep within the mine past the experimental area. In order to keep radon levels in the dome area and water purification system below 100 Bq m−3, fresh air is continually pumped at approximately 10 m3/min from outside the mine which generates a slight over-pressure in the Super-Kamiokande experimental area to minimize the entry of ambient mine air. A "Radon Hut" (Rn Hut) was constructed near the Atotsu tunnel entrance to house equipment for the dome air system: a 40 hp air pump with 10 m^3 min−1 /15 PSI pump capacity, air dehumidifier, carbon filter tanks, and control electronics. In autumn 1997, an extended intake air pipe was installed at a location approximately 25 m above the Atotsu tunnel entrance. This low level satisfies that goals of air quality so that carbon filter regeneration operations would no longer be required. Data processing Offline data processing is produced both in Kamioka and United States. In Kamioka The offline data processing system is located in Kenkyuto and is connected to Super-Kamiokande detector with 4 km FDDI optical fiber link. Data flow from online system is 450 kbytes s−1 on average, corresponding to 40 Gbytes day−1 or 14 Tbytes