Iconian gateway
An Iconian gateway or trans-dimensional gateway was a technology developed by the ancient Iconians 200,000 years ago – essentially a sophisticated transporter – the underlying principles of which remained well beyond Federation science.

These gateways allowed instantaneous travel over enormous distances, at least 70,000 light years. Physically the gateway appeared literally as a "door", either suspended in space or surrounded by a frame, through which was the destination. An individual could simply step through the gateway (as they would an ordinary door) and emerge on the other side. The gateway network allowed the Iconians to travel throughout the galaxy without the use of starships and was the chief means through which they controlled their vast empire, earning them a reputation among their primitive subject peoples as "Demons of Air and Darkness." (; )

Only two Iconian gateways were known to have survived the destruction of the Iconian civilization, both of which were later destroyed. The first was found in 2365 inside the only remaining structure on the surface of Iconia, and was accidentally activated by Lieutenant Commander Data as he was investigating the Iconian controls. The gateway apparently acquired possible destinations automatically, including the  and the Romulan warbird Haakona. The Enterprise-D away team used the gateway to escape, destroying the facility behind them to prevent it from falling into Romulan hands. ()

The second gateway was discovered in 2372 by the Dominion inside a stone ziggurat on the planet Vandros IV in the Gamma Quadrant. A group of renegade Jem'Hadar seized the gateway for themselves and re-activated it using equipment stolen from Deep Space 9. It was believed that they intended to use the gateway to foment an insurrection and mount a takeover of the Dominion. If they succeeded, the rogue Jem'Hadar would then gain the ability to move a force of any size onto any planet in the Federation or Dominion alike. Therefore, a joint Starfleet-Jem'Hadar team, led by Captain Benjamin Sisko and  Omet'iklan, successfully retook and destroyed the gateway. This second gateway generated some type of interference that prevented the functioning of energy weapons in its vicinity.

Previous to its destruction, the gateway led to Bajor (two different locations on the planet), Cardassia, Dozaria, Earth (Paris and San Francisco) and Volan III. ()

In demonstrating the gullibility of Harry Kim, Tom Paris told him that that morning, while on the bridge,  "received a hail from an Iconian scientist" who "claimed to have a trans-dimensional gateway that can take us anywhere in the galaxy." Kim didn't take the bait, however, when B'Elanna Torres confirmed Paris' story, adding that she figured they'd "be home by the end of the week," Kim began to fall for the story. ()

Appendices

Background information 
The computer voice of the Iconian gateway in "Contagion" was performed by an unknown actress.

One of the images in "Contagion" is clearly recognizable as the city hall buildings of Toronto, Ontario in Canada. As the city was not acknowledged during the episode, it is unknown whether it is supposed to be Toronto or simply stock footage meant to represent an alien city following Hodgkin's Law of Parallel Planetary Development.

Apocrypha
The discoveries of more Iconian gateways is the backbone of the Star Trek: Gateways novel miniseries.

In the novel The Devil's Heart, the Iconians developed the gateway technology through the use of an ancient stone called the Devil's Heart, which they called the "Dream Gem" and used it to activate the gateway network. However, their use of the technology led to suspicion and hatred being directed at the Iconains with a number of races bombarding Iconia in fear of the gateway technology. The gateways were used to transport Iconian survivors to a number of worlds which included DiWahn, Ikkabar and Dinasia. One of the last members of the race, Kanda Jiak, attempted to follow his compatriots during the desertion of Iconia, but the Heart instead used the gateway to transport him to , where he died.

In the novel series Star Trek: Dark Passions set in the mirror universe, the Klingon-Cardassian Alliance destroyed the Iconian homeworld rather than allow the gateway technology to fall into the hands of the Romulan Star Empire. Despite this being the case, by the 24th century, an Iconian portal was found by Intendant . In the late 24th century, a smaller portal device was discovered, which seemed to function as a miniaturized version of the gateway's technology.

According to Star Trek Online, the Iconian gateways constitute a galaxy-spanning network of thousands of gateways of various sizes, including ones large enough to admit fleets of starships. The network also extends towards the Andromeda Galaxy, though to what extent is unknown. It's also revealed that the Iconians do utilize spacecraft. These vessels are able to open ship-sized gateways at will, using them to travel great distances instead of traditional warp drives.

External link 
 

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Transporter technology