Fermats last theorem was an enigma until 1995 when Andrew Wiles proved it. But his proof is a little long and complicated. Fermat said that his proof was just a little to long to fit on the side of a page he wrote the theorem on. So the hunt for Fermats proof I think it still on, assuming that his proof was correct.
Fermats last theorem states that for any a,b,c there is no n larger then 2 that satisfies the equation
a^n + b^n = c^n
This only has to be proved for cases of a and b where they are coprime.