The atomic wars of the planet Mega were a series of nuclear conflicts that devastated the planet. According to Superman, no creature on the entire planet survived the wars. He was in error of course, because one man did survive the conflicts, a man named Hecla. (it is also later learned that a few Coluans had survived as well) He then built an android named Brainiac, and sent him into space to find specimens from other worlds to repopulate the mostly uninhabited planet. That's how Brainiac made his way to Earth, to collect earth specimens and take them back to Mega.
The current state of Mega now that the atomic wars are over is unknown, but it can be assumed that most of the planet is now uninhabitable due to atomic radiation.