Bajoran wormhole

The Bajoran wormhole was an artificial construct built by the beings known to the Bajorans as the Prophets, with a terminus in the Alpha Quadrant in the Denorios Belt of the Bajor system and another in the Gamma Quadrant, approximately 70,000 light-years distant. The wormhole was discovered in 2369 by Commander Benjamin Sisko and Lieutenant Jadzia Dax. (DS9 episode: "Emissary")

The wormhole was of major religious significance to the Bajorans, who called it the Celestial Temple.

History
The wormhole underwent a subspace inversion some time after its discovery. A Borg cube was destroyed by the event as it tried to pass into the Alpha Quadrant. 

In late 2373, Starfleet deployed a minefield at the mouth of the wormhole to stop more Dominion ships entering the quadrant. The Dominion tried to stop the USS Defiant (NX-74205) (I) during a battle at DS9. 

In April 2376 wreckage from the USS Aldebaran (Nebula class) destroyed near the wormhole kept triggering the opening of the terminus. This provided cover for Jake Sisko to secretly enter the wormhole. (DS9 novel: Avatar)

In August 2376 the Idran system, previously located three light-years distant from the wormhole's Gamma Quadrant terminus, inexplicably shifted into its immediate proximity. At that same time, the natives of the Idran system - the Eav'oq - reemerged from subspace meditation and revealed themselves, as well as their own reverence for the wormhole aliens, whom they called "Siblings". (DS9 novel: Rising Son)

Scientific research
In late 2371, Julian Bashir submitted a paper on the wormhole's effect on biological and chronological ages to the Journal of Quantum Medicine. (DS9 novel: Time's Enemy)