Some time in the mid 23rd century, Friendship One, which had been launched from Earth in 2067, entered Uxal's solar systems and went into orbit. The inhabitants of the planet salavage the probe, studied it, and were able to developed warp drive and antimatter weapons which had been previously unknown on their world. Unfortunately, a power failure in one of their missile silos resulted in the release of antimatter radiation, contaminating their atmosphere and killing off a large portion of the population.
Following the disaster, the planet entered into what amounted to a nuclear winter, and suvivors took refuge in underground complexes where they were exposed to the radiation and suffered severe health problems.
In 2377 the Federation starship USS Voyager was sent by Starfleet to locate the whereabouts of the missing probe, Friendship One. Visiting Uxal and learning of the damage done to the planet, the crew neutralized the radiation, and restored the planet's atmosphere. The surviving Uxali then migrated to the surface.