The happy ending problem
So easy to describe, yet so hard to prove.
Mathematics is full of stories of geniuses toiling away in isolation before presenting the world with a monumental result that takes years for everyone else to understand. A famous example is Fermat's last theorem, a 350-year old problem that was solved in the 1990s by Andrew Wiles after years of secretive work.
Mathematicians are working hard to understand an impenetrable proof of the famous ABC conjecture.