Bajoran wormhole

"I remember when I first saw the gate of the Celestial Temple – I was on the Promenade. When it burst into view, this whirlpool of color and light, the people all around me were in awe. They said they could feel the love of the Prophets washing over them."
 * - Kai Winn Adami, 2375

The Bajoran wormhole was the only stable wormhole known to exist in the Milky Way Galaxy. It crossed some seventy thousand light years, with one terminus located in the Denorios belt of the Bajoran system in the Alpha Quadrant, and the other at a point near the Idran system in the Gamma Quadrant.

The wormhole was the home of extra-dimensional beings who were worshiped by the Bajorans as gods, known as the Prophets, and therefore was sometimes referred to as the Celestial Temple in that context. Kor, a Klingon, once called the wormhole the "Eye of Destiny". In the Gamma Quadrant, the wormhole is known as "the anomaly" by the Jem'Hadar, and to the Skrreea, according to their mythology, as the Eye of the Universe. The wormhole is also known to the Changelings as the "passageway". 

It was theorized that the wormhole was formed by unique particles, called verterons, that were self-sustaining in nature. Highly elevated levels of neutrinos were also commonly detected prior to the appearance of the wormhole's event horizon, and as a vessel traversed the passage. It was believed that verteron nodes played a large part in keeping the wormhole stable. 

History
Bajoran historical records indicate that the wormhole had most likely been in existence for at least the past ten thousand years by the late 24th century. The nine Orbs which had appeared on or near Bajor in that time, and are revered as active links between the Bajoran people and the Prophets, are thought to have come from the wormhole. There were also several unexplained incidents over the centuries that were later attributed to the wormhole after its discovery. In the 22nd century, at least two Bajorans encountered the wormhole: Kai Taluno, whose ship was disabled inside the Denorios belt for several days and reported that "the heavens opened up and nearly swallowed the ship."; and Akorem Laan, whose lightship was damaged in an ion storm, drifted into the Denorios belt, and was then swallowed by the wormhole, where it, and Akorem, would stay with the Prophets until 2372. In 2337, an unknown and unpiloted alien ship was found in the Denorios belt, with a shapeshifting being aboard, who would later be named Odo.

Due to the Denorios belt being generally avoided by interplanetary traffic, the wormhole remained undiscovered until 2369, shortly after the end of the Cardassian Occupation of Bajor. When Starfleet took over the operation of the space station Terok Nor in orbit of Bajor, Commander Benjamin Sisko and science officer Lieutenant Jadzia Dax investigated the many reported anomalies in the belt aboard the USS Rio Grande, and became the official discoverers of the wormhole. Shortly thereafter, the station, renamed Deep Space 9, was relocated at a position near the terminus of the wormhole. 

Keiko O'Brien's classroom aboard Deep Space 9 in 2369 featured a wall chart with an image of the Bajoran wormhole beside other Federation starships. She discussed the history and scientific roots of the wormhole in her lessons, though in a secular fashion, which angered some the Bajoran devout. 

On one occasion, the wormhole transported a runabout and two crew members to a parallel universe in 2370. This was due to a plasma injector leak which caused the runabout to enter the wormhole with an only partially-collapsed warp field. The two crew members, Major Kira Nerys and Doctor Julian Bashir, were later able to return to their own universe by recreating the accident and returning through the wormhole. 

The wormhole had an enormous impact on interstellar politics in the Alpha Quadrant. In the first two years, many races eagerly sent explorers and freighters through the wormhole to open relations with new trading partners, and to establish new colonies. However, the wormhole also brought the powers of the Alpha Quadrant into contact with the Dominion, a powerful government that controlled a large swath of territory in the Gamma Quadrant. 

In 2371, Starfleet initiated a joint scientific project with the Bajoran and Cardassian governments to establish a trans-wormhole subspace communications relay, to make it possible for ships in the Gamma Quadrant to contact home, as well as to provide early warning of any Dominion attack. The mission was complicated by a warning from Bajoran Vedek Yarka, who cited Trakor's Third Prophecy, about the potential destruction of the Celestial Temple. This prophecy was unexpectedly validated soon after with the approach of a previously-unknown, long-period comet. This comet was infused with silithium, which had the potential to permanently destabilize the wormhole if it interacted with the verteron particles in the passage. Unable to alter the comet's trajectory, a shuttlepod from the USS Defiant (2370) used a deflector shield to prevent the comet's material from contaminating the wormhole. During the transit, trace amounts of silithium inadvertently leaked through the shields, however, the small amounts did not destabilize the wormhole, but instead created a subspace filament inside the wormhole that allowed normal communication between the quadrants. 

In 2373, when a Dominion invasion became inevitable, Starfleet ordered the entrance to the wormhole be collapsed. Doctor Lenara Kahn of the Trill Science Ministry had devised a means, using a phase-conjugate graviton beam to seal the wormhole without causing damage to it or harming the Prophets. Deep Space 9 was able to rig the phase-conjugate beam by remodulating the station's deflector grid frequencies and projecting it through its graviton emitters. Unfortunately, when executed, it was discovered that the emitters were sabotaged and, as a result, the phase-conjugate graviton beam had the exact opposite effect on the wormhole, and rendered the wormhole's spatial matrix even more stable than before, so stable in fact that not even trilithium explosives could destroy the wormhole. Thus, Starfleet was unable to prevent a series of Dominion fleets from entering the Alpha Quadrant. 

A few months later, Starfleet was forced to blockade the wormhole with self-replicating mines in order to prevent the growing Dominion threat to the Alpha Quadrant. Deep Space 9 was subsequently lost to the Dominion in the opening battle of the Dominion War, but the minefield was successfully planted, preventing further Dominion ships from traversing the wormhole. The minefield was eventually destroyed while the Federation attempted to retake the station. The Dominion fleet that was passing through the wormhole at the time was destroyed by the Prophets however, since the Prophets' Emissary chose to die trying to stop the Dominion fleet rather than heed their will. After this event, no further traffic passed through the wormhole until the end of the war.

With the wormhole considered impassable, the Dominion introduce a new breed of Jem'Hadar, the Alphas, in 2374. 

The wormhole was apparently destroyed in late-2374, when Dukat, acting as a conduit for the exiled Pah-wraiths, attacked the Orb of Contemplation, which was then housed on Deep Space 9. Following Dukat's attack, all of the Orbs went dark, and the wormhole itself disappeared. Dukat's aim was to force the Prophets out and make the wormhole passable again for Dominion reinforcements, although he did not anticipate that the wormhole would collapse. 

The disappearance of the wormhole and the apparent loss of contact with their deities caused great consternation and fear among the Bajorans, as well as the rise of the Cult of the Pah-wraiths. The wormhole remained closed for the next three months, until Benjamin Sisko, acting as the Emissary of the Prophets, discovered the Orb of the Emissary on Tyree in early-2375. The discovery of the previously-unknown tenth Orb caused the re-opening of the wormhole. 

Later that year, Dukat told Damar that he had no regrets in failing to reopen the wormhole by killing the Prophets. 

Jake Sisko, along with Colonel Kira Nerys, watched the wormhole opening from the Promenade after Captain Sisko's encounter with Dukat in the Fire Caves. 

Background information
The visual effect for the wormhole was created by Rhythm & Hues.

Apocrypha
In the novelization of, immediately after Dukat is rescued from the wormhole, he attempts to lay claim to it based on Cardassia's long-standing Occupation of Bajor, before Sisko coolly informs him that he has reached an agreement with the Prophets on behalf of the Federation and Bajor that they will control the wormhole, and anyone trying to go through it without the permission of those sides would find themselves lost somewhere mid-way between the wormhole's entrances (Although given later events with various sides using the wormhole this may have been at least partly a bluff).

In the Deep Space Nine book trilogy Millennium, there was a second Bajoran wormhole, a Pah-wraith wormhole that opened when the three Orbs of the Pah-wraiths were brought together in Quark's. The second wormhole actually opened inside of Quark's, ripping Deep Space 9 to pieces. The opening of the second wormhole nearly spelled Armageddon for the entire universe as, in an alternate future, Kai Weyoun triggered a gravimetric explosion that brought both wormholes together, causing them to connect to each other and swallow the entire universe. However, thanks to the actions of Ben Sisko, the crew of the USS Defiant, and Elim Garak, Quark, and Jake Sisko (who were close enough to the wormholes to be absorbed directly into the dimension of the Prophets and the Pah-Wraiths when the wormholes opened rather than being consumed), these events were undone when they took advantage of a temporal anomaly inside the wormhole dimension- putting them in touch with DS9 prior to its destruction and in the last days of the Occupation- to alter the events leading up to the destruction of Deep Space 9- without altering anything that would prevent them being in a position to make those changes-, culminating in them destroying a ship from that alternate future and using the energy to prevent the second wormhole forming in the first place.

In the DS9 relaunch series and the subsequent Typhon Pact series, a renegade branch of the Typhon Pact- an alliance of the Federation's enemies- manage to create an artificial wormhole that 'connects' to the Bajoran wormhole so that they can enter the Gamma Quadrant in secret in the novel Raise the Dawn, believing that Dominion technology will help them develop their own version of the quantum slipstream drive. This artifical wormhole is destroyed by Kira Nerys, but her attack apparently costs her her life and closes the Bajoran wormhole, leaving Odo trapped in the Alpha Quadrant.