FERMAT'S LAST THEOREM
In the margin of his copy of Claude-Gaspar Bachet's translation of the famous Arithmetica by Diophantus, , Pierre de Fermat wrote:
"It is impossible for a cube to be the sum of two cubes, a fourth power to be the sum of two fourth powers, or in general for any number that is a power greater than the second to be the sum of two like powers. I have discovered a truly marvelous demonstration of this proposition that this margin is too narrow to contain."
That was in 1637. It took 361 years to prove this conjecture. The journey produced some of the most fascinating discoveries in mathematics.