Fatto generale: 6603 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 6603 is a number whose square and cube use different digits.
Trivia: 6603 is an uninteresting number.
Data storica: January 28th is the day in 1855 that a locomotive, on the Panama Railway, runs from the Atlantic Ocean to the Pacific Ocean for the first time.