The deepest, and indeed largest, operating neutrino observatory is the Super-Kamiokande, a joint US-Japanese research facility located 1,000 m (3,280 ft) below ground in the Mozumi Mine (Kamioka Mining and Smelting Co.) in Hida city, Gifu, Japan. It consists of a vast cylinder measuring 40 m (131 ft) in height and 40 m (131 ft) in diameter, which is filled with ultra pure water. The walls are covered with 13,000 sensitive light detectors called photomultiplier tubes, which watch the water for Cerenkov light – the telltale sign that a particle has passed through the water. Super-K, as it is also know, started observation in 1996.