The first letters (at least one, often but not always two) of any ICAO airport code represent the country it's in, and by convenient coincidence, the only three

The first letters (at least one, often but not always two) of any ICAO airport code represent the country it's in, and by convenient coincidence, the only three unused letters are I, J and X. I personally would probably have plumped for X as in extraterrestrial, but Xxxx codes are already frequently used for unofficial codes like train stations, so maybe someday Mars will indeed be J or JZ. https://en.wikipedia.org/wiki/ICAO_airport_code https://en.wikipedia.org/wiki/ICAO_airport_code