Fatto generale: 21998 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21998 is a boring number.
Trivia: 21998 is an unremarkable number.
Data storica: March 23rd is the day in 1989 that Stanley Pons and Martin Fleischmann announce their discovery of cold fusion at the University of Utah.