lAn air shower traveling along an exactly vertical, downward trajectory traverses ~1,000 g/cm2 in reaching sea-level. lFollowing the above convention, the depth of shower maximum is denoted "Xmax". With a value of about 500 g/cm2 at 1015 eV, the average Xmax for cosmic ray showers increases by 60-70 g/cm2 for every decade of energy.