  We propose a novel approach for observing cosmic rays at ultra-high energy
($>10^{18}$~eV) by repurposing the existing network of smartphones as a ground
detector array. Extensive air showers generated by cosmic rays produce muons
and high-energy photons, which can be detected by the CMOS sensors of
smartphone cameras. The small size and low efficiency of each sensor is
compensated by the large number of active phones. We show that if user adoption
targets are met, such a network will have significant observing power at the
highest energies.
