Fatto generale: 8991 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 8991 is the smallest number so that it and its successor are both the product of a prime and the 5^{th} power of a prime.
Trivia: 8991 is an uninteresting number.
Data storica: August 12th is the day in 1793 that the Rhône and Loire (Lêre) départments are created when the former département of Rhône-et-Loire is split into two.