  The very-forward energy production in hadron collisions is of paramount
importance for the understanding of ultra-high energy cosmic ray air showers.
The CASTOR calorimeter of CMS is located at $-6.6 < \eta < -5.2$ in the
phase-space where the peak of energy is deposited at LHC. The composition and
characteristics of the particles in this phase-space have a determining impact
on the formation of air shower cascades. An overview of various energy
measurements performed with CASTOR is reported and possible implications for
cosmic ray physics are outlined.
