Unasked-for experimentation involving miscroscopic tektites means that the immortal Mitch Shelley regenerates every time he dies, the tektites in his bloodstream resurrecting him with a new set of powers [much like Multi-Man] which are often a response to the manner of his death. He is opposed to Vandal Savage, and will be so for many years. He was once thought to be the reincarnation of the Immortal Man, but this has since been discovered to be untrue. It is possible that his immortality was granted by the same meteor that did the same for Savage and Immortal Man, though this has never been confirmed.
It is known that Mitch will survive at least as far as into the 853rd century, where he will be the adult leader of the Justice Legion B (that era's Teen Titans) and strategic guide of the Justice Legion A. Although it seemed that he was definitely killed by Vandal Savage in that era, he returned shortly afterwards with renewed life once again.