We propose a novel approach for observing cosmic rays at ultra-high energy ( > 10 18 eV) by repurpos- ing 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 num- ber of active phones. We show that if user adoption targets are met, such a network will have significant observing power at the highest energies.