General fact: 138 is essendon's losing margin against Adelaide in round 10 of the 2006 AFL season.
Mathematical fact: 138 is the smallest product of 3 primes, such that in base 10, the third prime is a concatenation of the other two.
Trivia: 138 is the rank of Mongolia in world population.
Historical date: May 17th is the day in 1902 that Greek archaeologist Valerios Stais discovers the Antikythera mechanism, an ancient mechanical analog computer.