The GKZ effect is based on sound theoretical ideas and ought to be there. Nobody has been able to find anything wrong with the experimental observations, so other options have to be taken seriously. In past, whenever experimental data contradict established theories, rapid progress has followed. One hundred years ago, physicists similarly faced several puzzles, the solution of which led to quantum theory and relativity. For this reason, we think it is essential to collect additional observations of the highest energy cosmic rays.
Cosmic rays approaching the earth soon interact in nuclei within the atmosphere.
We live under a layer of air amounting to about 1000 grams of air covering every square cm of the earth's surface. Luckily for all of this, the atmosphere shields us from cosmic rays. Unluckily for cosmic ray physicists, this means we cannot make direct observations of cosmic rays at sea level. When the original cosmic ray is sufficiently energetic, the result is a huge shower of subatomic particles striking the earth. The total number of particles in the shower is proportional to the primary particle's energy.