The Millennium Prize Problems are seven problems in mathematics that were stated in 2000. These problems are old problems. Nowadays, six of the seven problems remain unsolved. A correct solution to any of the problems results in a one million dollar prize. $1,000,000 is about 800,000 euros.
The only solved problem is the so-called Poincaré conjecture (now is called Poincaré theorem as it has already been proven). The problem was originally posed by the French mathematician Poincaré in 1904, and it wasn't solved until a century later by the Russian mathematician Grigori Perelman. He was award in 2010 with the $1,000,000 prize but... he declined the prize! (wow!)
Perelman also declined the Field Medal (the most relevant prize in mathematics). He said that:
"Everybody understood that if the proof is correct, then no other recognition is needed. I'm notinterested in money or fame''
To conclude, here you have the list of the 6 problems that still remain unsolved (just in case you want to think about them):
- The Riemann Hypothesis
- Yang-Mills Theory and the Mass Gap Hypothesis
- The P vs. NP problem
- The Navier-Stokes Equations
- The Birch and Swinnerton-Dyer Conjecture
- The Hodge Conjecture
K. Devlin. The Millennium Problems. Basic Books (2002)