  Cosmic rays are energetic charged particles from extraterrestrial sources,
with the highest energy events thought to come from extragalactic sources.
Their arrival is infrequent, so detection requires instruments with large
collecting areas. In this work, we report the detection of an extremely
energetic particle recorded by the surface detector array of the Telescope
Array experiment. We calculate the particle's energy as 244 +- 29 (stat.)
+51,-76 (syst.) exa-electron volts (~40 joules). Its arrival direction points
back to a void in the large-scale structure of the Universe. Possible
explanations include a large deflection by the foreground magnetic field, an
unidentified source in the local extragalactic neighborhood or an incomplete
knowledge of particle physics.
