  We report on the detection of extremely narrow Feshbach resonances by
employing a Mott-insulating state for cesium atoms in a three-dimensional
optical lattice. The Mott insulator protects the atomic ensemble from high
background three-body losses in a magnetic field region where a broad Efimov
resonance otherwise dominates the atom loss in bulk samples. Our technique
reveals three ultra-narrow and previously unobserved Feshbach resonances in
this region with widths below $\approx 10\,\mu$G, measured via
Landau-Zener-type molecule formation and confirmed by theoretical predictions.
For comparatively broader resonances we find a lattice-induced substructure in
the respective atom-loss feature due to the interplay of tunneling and strong
particle interactions. Our results provide a powerful tool to identify and
characterize narrow scattering resonances, particularly in systems with complex
Feshbach spectra. The observed ultra-narrow Feshbach resonances could be
interesting candidates for precision measurements.
