The 70th Hunger Games was the Hunger Games that took place four years prior to the start of the series. This Hunger Games was won by Annie Cresta, the female District 4 tribute. These Games are mentioned in Catching Fire, though little is known about them.
It all went wrong when the Gamemakers wanted to liven things up becuase only a few tributes had died after a few days of the games. In response to this problem, the Gamemakers started an earthquake to break a dam overlooking the arena. The dam's collapse produced a massive flood that flooded the entire arena, that drowned most of the other tributes. Annie knew how to swim very well, being from District 4, and she lived.
Annie Cresta was the best swimmer in her Games, and was the only survivor of the flood as she is from District 4, securing her as the victor of the 70th Hunger Games. She was never the same afterwards, as the trauma made her mentally unstable.