  Regarding ultrahigh-energy cosmic rays (UHECRs) as a probe and extra
dimensions as a possible ingredient of the fantastic ultrahigh-energy world, we
discuss possible interplay between them. On the one hand small extra dimensions
and Kaluza-Klein bursts present a feasible way to the origin of UHECRs. On the
other hand large extra dimensions may change various interactions between
particles at ultrahigh energies and therefore impact on air showers created by
UHECRs. Conversely, UHECR data may tell us secrets veiled in extra dimensions.
