Fatto generale: 21870 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21870 is an uninteresting number.
Trivia: 21870 is an unremarkable number.
Data storica: November 16th is the day in 1973 that U.S. President Richard Nixon signs the Trans-Alaska Pipeline Authorization Act into law, authorizing the construction of the Alaska Pipeline.