The Yaphit Pass was a stable wormhole between Etrusca and Thule. As Thule lay some distance above the galactic plane, projection drive would not provide access to the distant planet. The Yaphit Pass provided the only stable passage to Thule.
In 430 IE, the Thulians themselves collapsed the pass during the Liberation of Amargosa, effectively leaving the Compact and isolating the distant world from the rest of humanity.
Some time before 432, the Metis government discovered that the wormhole did not completely collapse but left a remnant. Though dangerous, a ship could navigate it. In 432, as Metis, Amargosa, and Farigha seceded from the Compact, the new Metisian Navy sent the Bova to carry Jayne Best to Thule for treatment of her injuries.
Appearances: The Amortals, Storming Amargosa, Suicide Run