The dip is a feature in the diffuse spectrum of UHE protons in energy range 1× 10^18 - 4× 10^19 eV, which is caused by electron-positron pair production on CMB photons. Calculated for power-law generation spectrum with index γ_g=2.7, the shape of the dip is confirmed with high accuracy by data of Akeno - AGASA, HiRes, Yakutsk and Fly's Eye detectors. The predicted shape of the dip is robust: it is valid for the rectilinear and diffusive propagation, for different discretenesses in the source

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... tribution, for local source overdensity and deficit, for source inhomogeneities on scale ℓ 100 Mpc etc. Below the characteristic energy E_c ≈ 1× 10^18 eV the spectrum of the dip flattens for both diffusive and rectilinear propagation, and more steep galactic spectrum becomes dominant at E < E_c. The energy of transition E_ tr < E_c approximately coincides with the position of the second knee E_2kn observed in the cosmic ray spectrum. The critical energy E_c is determined by the energy E_ eq = 2.3× 10^18 eV, where adiabatic and pair-production energy losses are equal. Thus, position of the second knee is explained by proton energy losses on CMB photons.