knee may be explained either as of extragalactic and or Galactic origin. Since the Larmor radii of the particles with the energy in EeV region become larger than the thickness of the Galactic disk, it is likely that their sources are extragalactic. The interesting aspect of the extragalactic CRs with energies exceeding 50 EeV are the energy losses due to the interactions with cosmic microwave background. These energy losses[^2] constrain detected UHECRs to have been produced in the sources within 100 Mpc. This distance restriction is known as Greisen-Zatsepin-Kuzmin (GZK) cutoff ([@greisen], [@zacepin]).

Earth’s atmosphere absorbs high energy cosmic rays and so they reveal their existence on the ground only by indirect effects such as ionization and showers of secondary charged particles covering areas up to many km$^2$. The energy flux of CRs is rapidly decreasing with their increasing energy. We observe one particle per m$^2$ per year at energies of $10^{15}$ eV but only one particle per km$^2$ per year at energies of $10^{18}$ eV. Thus, we need a large detector to find and measure these rare events. In the next decade the Pierre Auger Observatory should be able to collect several hundreds of events above the GZK cutoff, 