Metaphasic radiation is a form of energy which, under certain conditions, reverses the aging process of most humanoid species, thereby prolonging the life of those who are exposed to it. Metaphasic radiation also has remarkable regenerative and rejuvenative properties, increasing the metabolism and energy levels of those irradiated by it. However, this increased health relies on constant exposure to the radiation; injuries healed by the radiation would eventually return to their previous state once the subject was no longer exposed to the radiation.
High concentrations of metaphasic radiation were discovered to be contained within the rings of the planet Ba'ku in the Briar Patch in 2375. An offshoot race of the Ba'ku, the Son'a, were desperate to harvest the radiation so as to expand their lifespans, having severely damaged their bodies while attempting to artificially extend their lives after being forced off the planet. However, since the Ba'ku planet was in Federation territory, they enlisted the help of Starfleet Admiral Matthew Dougherty to have the Federation Council agree to have the radiation harvested, under the pretense of medical purposes. This would have meant relocating the Ba'ku people, all six hundred of them, to another world, as the loss of the radiation would mean making Ba'ku uninhabitable. Ultimately the USS Enterprise-E was able to foil the plan, as the crew felt that the moral crimes that would be committed to acquire the radiation were not worth the medical advances that would result from closer study of the radiation.
Several Enterprise crewmembers experienced the regenerative process while aiding the Ba'ku. The cells in the eyes of engineer Geordi La Forge had begun to regenerate, allowing him to see like ordinary Humans. Lieutenant Commander Worf also re-entered Jak'tahla, the Klingon version of puberty. These changes were apparently reversed after the crew departed from Ba'ku and they were no longer regularly exposed to the radiation. (Star Trek: Insurrection)