Fermat's Last Theorem.

Fermat's Last theorem:

It states an extension of Pythagorean Theorem to higher dimensions is not possible. In simple terms, we can't merge two cubes into a single cube unlike two squares can be merged into a single square. Proving Pythagorean theorem is very simple, but proving Fermat's Last theorem took 350 years until Andrew Wiles successfully proved it. A single proof which almost covers all possible areas of Mathematics, makes this theorem and proof very interesting.

http://en.wikipedia.org/wiki/Fermat's_Last_Theorem