Barzan wormhole

The Barzan wormhole was a wormhole with one end initially located near the planet Barzan II in the Alpha Quadrant and one end that moved unpredictably to points through the galaxy, including the Gamma and Delta Quadrants. 

History
The wormhole provided the Barzans with their first true natural resource, an opportunity which they decided to capitalize on by opening up negotiations for use of the wormhole. Present at the negotiations were representatives for the Federation, the Caldonians, the Chrysalians (represented by Devinoni Ral), and, later, the Ferengi.

In the Alpha Quadrant, the wormhole became visible every 233 minutes, due to radiation buildup in the accretion disk. A Barzan probe sent through the wormhole discovered that the Gamma Quadrant terminus was located beyond the Denkiri Arm, a distance that at warp 9 would have taken nearly a century to cover.

During the negotiations, the USS Enterprise (NCC-1701-D) conducted scientific studies of the wormhole, sending Lieutenant Commanders Data and Geordi La Forge through in a shuttlecraft, followed by two Ferengi, Arridor and Kol. Emerging from the wormhole, Data and La Forge discovered that, as opposed to arriving in the Gamma Quadrant as expected, they had emerged in sector 3556 of the Delta Quadrant. They quickly realized that although the terminus in the Alpha Quadrant was stable, the other end shifted positions periodically, and that both ends would eventually destabilize. Data and La Forge attempted to explain the situation to the Ferengi, who refused to accompany the Starfleet officers back through the wormhole. Data and La Forge returned to the Alpha Quadrant, but Arridor and Kol stayed, and they watched in shock as the wormhole destabilized, stranding them in the Delta Quadrant. 

In 2373, the starship USS Voyager, which had become stranded in the Delta Quadrant because of the Caretaker's array, discovered Arridor and Kol living on a planet, in a region called Takar, posing as gods by using the religion of the local inhabitants to exploit them. The two lived lavish lives while the people were mostly in squalor. Captain Kathryn Janeway did not want to let the Ferengi continue with their gross exploitation of the inhabitants; at the same time, she and her crew tried to find a way to make the wormhole appear long enough for Voyager—and the two Ferengi they planned to take with them—to go through.

Voyager bombarded the area of the subspace instability created by the wormhole with verteron particles, which they hoped would amplify and polarize the instability, attracting the wormhole so they could travel through it back to the Alpha Quadrant. This was a risky undertaking, however; many different factors could deflect the wormhole from their sector, such as gravimetric shifts or solar flares.

Escaping Voyager in their shuttle, the two Ferengi initiated a high-energy graviton pulse, which changed the rotational momentum of the wormhole, destabilizing it. Although the gravitational eddy of the wormhole pulled the shuttle in, the graviton pulse had knocked the wormhole completely off its subspace axis, causing the wormhole to jump around erratically on both ends before disappearing altogether. This once again ended Voyager's hopeful attempts to return home. 

Background
The TNG episode was the first show to use the term "Delta Quadrant" and establish its extreme distance—and the tendency for spatial phenomena, such as wormholes, to be unstable.

Apocrypha
In the DS9 novel A Stitch in Time, Cardassians had suspected the existence of the wormhole nearly a decade before it was discovered, and Elim Garak studied it while he was at the Bamarren Institute for State Intelligence.