Metaphasic radiation was a form of energy which, under certain conditions, reversed the aging process of most humanoid species, thereby prolonging the life of those who are exposed to it. Metaphasic radiation also had remarkable regenerative and rejuvenative properties, increasing the metabolism and energy levels of those irradiated by it.
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. 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 600 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.
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. (Star Trek: Insurrection)