[en] We describe a 125 m diameter spherical detector containing 1 Mt of water, capable of high rate observation of atmospheric neutrino events (30000/y). The ring imaging Cherenkov (RICH) technique is used to measure velocity, momentum and direction of particles produced by neutrinos interacting in water. The detector will be sited outdoors (under a 50 m water shield) in a natural (further excavated) pit, probably in Sicily. Spherical reflecting mirrors focus Cherenkov light produced by secondaries from interacting neutrinos. Photons are detected by 5310 hybrid photodiodes (HPDs) of 1 m diameter each with 396 pads of 45x45 mm² on the photocathode surface, demagnified to 9x9 mm² on the silicon sensor. For most tracks the ring width will be dominated by multiple scattering which should allow momentum to be determined. Hadrons of momentum p≤5 GeV/c can be measured with σ_p/p≤7% and muons of p≤32 GeV/c with σ_p/p≤1%. The ring center determines track direction with σ_θx, σ_θy∼6 mrad. Track reconstruction in water will require time resolution σ_t<1 ns. Detection of oscillating muon signals (disappearance) is feasible with atmospheric neutrinos and precision measurement of oscillation parameters is feasible for 10^-2≤Δm²≤10^-4eV². Tau or sterile neutrino appearance experiments are also possible with atmospheric neutrinos. Other physics topics addressable with this detector are proton decay, supernova detection and search for astrophysical neutrino sources. A first test module of 3 tons water and 120 PMs (32 mm phi) will operate (5/99) with 1-3 GeV muons in a CERN-PS test beam to verify the momentum algorithm. A second (6 m)³ test module with 216 tons of water and 25 HPDs (0.25 m phi) is designed to observe full multi-track images and test pattern recognition and ring reconstruction algorithms