  We propose an experiment to test the hypothesis that the reported anomaly on
atmospheric neutrino fluxes is due to nu_mu <-> nu_x oscillations. It will rely
both on a disappearance technique, exploiting the method of the dependence of
the event rate on L/E, which was recently shown to be effective for detection
of neutrino oscillation and measurement of the oscillation parameters, and on
an appearance technique, looking for an excess of muon-less events at high
energy produced by upward-going tau neutrinos. The detector will consist of
iron planes interleaved by limited streamer tubes. The total mass will be about
30 kt. The possibility of recuperating most of the instrumentation from
existing detectors allows to avoid R&D phases and to reduce construction time.
In four years of data taking, this experiment will be sensitive to oscillations
nu_mu <-> nu_x with Delta m^2 > 10^-4 eV^2 and a mixing near to maximal, and
answer the question whether nu_x is a sterile or a tau neutrino.
