691 is a prime number.
691 is the only known prime number that is a square when turned upside down (169) and another square when reversed (196).
691 is the smallest prime that can be written as the sum of thirteen consecutive primes.
691 is the (negative) numerator of the 12th Bernoulli number (-691/2730).
As conjectured by Ramanujan and proved by G. N.Watson, Ramanujan's tau function is divisible by 691 for almost all positive integers.
Source: Prime Curios!