  By invoking small extra dimensions as a good energy bearer, a new scenario
for understanding the origin of ultrahigh-energy cosmic rays (UHECRs), from
both the bottom-up and the top-down viewpoints, is proposed. We explore the
possibility of generating UHECRs via Kaluza-Klein (KK) bursts, a violent energy
transfer from extra dimensions to ordinary dimensions through collisions
between KK modes, in particular, within clumps of KK modes. The possible scales
of these clumps range from the astronomical, e.g. KK stellar compact objects,
to the microscopic, e.g. "KKonium". Advantages of this KK burst model of UHECRs
and possible signatures of clumped KK modes are discussed.
