One cold clear winter night in 1991, a single subatomic particle came hurtling through the atmosphere destroying atoms, causing the air to glow, and in general behaving like a normal cosmic ray, except for its energy. This single cosmic ray had an energy of nearly 1020 electron volts, or nearly 50 joules! To put it into perspective, that is roughly equivalent to a 1 kg mass moving at 10 m/s, or a 90 MPH fastball. The energy of a fastball is shared by 1025 atoms. Imagine one atomic nucleus having all that energy! This was the most energetic cosmic ray ever recorded and it was observed by the University of Utah’s Fly’s Eye detector.

The Fly’s Eye detector is a detector that "looks" for cosmic rays. Of course cosmic rays are subatomic particles and are far too small to actually be seen. What the detector actually sees is the trail left by the cosmic ray as it bumps into the nitrogen in the atmosphere. The Fly’s Eye detector is a series of mirrors, each pointed in a different direction. Because the mirrors are pointed all around, the Fly’s Eye detector can "see" in all directions, the same as a fly. Which is why it is named the Fly’s Eye detector.