The nebula was roughly 16 AUs across and greenish-blue in color. It contained subspace rifts, graviton sheers, methane, and sirillium. It was also the only known source of a the OFT isotope of laracite. Powerful ion storms were a regular occurrence inside the nebula. Due to these storms, the nebula had been considered unsafe for starships since it was first charted in the early 23rd century.
In 2407, the USS Nautilus was sent to the nebula to investigate the possibility of prehistoric contact between aliens in the nebula and ancient Japan. A class M planet was discovered inside, orbiting a G-type star. The Nautilus attempted to send an away team to the planet, but their shuttle crashed on it in an ion storm. A rescue shuttle and two probes were subsequently lost in the nebula, followed by a bird-of-prey which attempted to abduct the Starfleet survivors while investigating Federation activity in the area. Further attempts to enter the nebula were deemed too dangerous.
While searching for a way to retrieve their away teams safely, the Nautilus discovered a number of subspace distortions on the planet, as well as conduits conducting polaric ion energy radiating from a central nexus on the main island. This network was determined to be responsible for the ion storm activity in the nebula as well as the polaric ion discharges in the atmosphere of the planet itself. The crew theorized that the network could be a living entity, but was unable to determine how to communicate with it. Admiral Quinn ordered them to destroy the entity by targeting its nexus with a tricobalt device, but these orders were later rescinded.
The nexus was actually the subspacial manifestation of the Sun Queen Himiko, a polaric ion energy based lifeform trapped on the planet after attempting and failing to transfer itself to a new human host. When the entity was destroyed, the resulting polaric energy explosion sent a shockwave through the nebula that propogated at warp 4.96 and caused suspace ruptures, rendering it impassible and extremely hazardous for the better part of a decade. ("Survivors of Yamatai")