  The planned LBL experiments will be able to prove the hypothesis of flavor
oscillation between muon and tau neutrinos. We explore the possibility of a
second generation long baseline experiment at very long baseline, i.e. L in the
range 5000-7000 km. This distance requires intense neutrino beams that could be
available from very intense muon beams as those needed for $\mu$ colliders.
Such baselines allow the study of neutrino oscillations with $E/L \approx
2\times 10^{-3} eV^2$ with neutrinos of energy $E_\nu \approx 10 GeV$, i.e.
above tau threshold. Moreover, matter effects inside the Earth could lead to
observable effects in $\nu_e \to \nu_\mu$ oscillations. These effects are
interchanged between neutrinos and antineutrinos, and therefore they can be
tested by comparing the oscillated spectra obtained running the storage ring
with positive and negative muons.
