An extreme-energy cosmic ray (EECR) is an UHECR with energy exceeding 5 × 10 eV (about 8 joule, or the energy of a proton traveling at ≈ 99.999 999 999 999 999 999 98 % the speed of light), the so-called Greisen–Zatsepin–Kuzmin limit (GZK limit). This limit should be the maximum energy of cosmic ray protons that have traveled long distances (about 160 million light years), since higher-energy protons would have lost energy over that distance due to scattering from photons in the cosmic microwave background (CMB). It follows that EECR could not be survivors from the early universe, but are cosmologically "young", emitted somewhere in the Local Supercluster by some unknown physical process.