Fatto generale: 21409 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21409 is an unremarkable number.
Trivia: 21409 is an uninteresting number.
Data storica: August 12th is the day in 2007 that the bulk carrier M/V New Flame collides with the oil tanker Torm Gertrud at the southernmost tip of Gibraltar, ending up partially submerged.