Slipgate

Slipgates in Quake are devices built by the humans of Earth before they were stolen and used by Quake as a way of teleporting onto Earth. They were made to transport humans and cargo from one place to another instantaneously. Slipgates worked by generating earthquakes that would disrupt the time continuum.

As they were possessed by Shub-Niggurath to transport her spawn of daemonic aliens from the other dimensions, the gateway was still open when Ranger went through. Other than their creators, they are exactly the same as a Teleporter for lore purposes, but work differently in terms of mechanics.

Slipgates are the Level Exits for idBase levels. In the level after the idBase level there is an entrance Slipgate to keep continuity of the levels. These "Starting Slipgates" are identical to the pads the player stands on in the full-sized "Exit Slipgate". Exit Slipgates also have a static texture in the front and animated lighting on the sides, plus an upper lip that is similar in shape to the pad one stands on to leave the level. In Deathmatch games, the Slipgate has a larger border around it so the player does not accidentally step into it.

Trivia

 * Slipgates were conceived to make the levels feel less disjointed.
 * According to John Romero, the Slipgates were originally supposed to have Cray supercomputers powering them. This is due to Romero wishing to make the idBase levels in modern day settings. They would also be given a deal on a Cray supercomputer for their product placement. Whilst in the middle of adding the supercomputers to the game, Cray was bought out by SGI in February, 1996. Romero added the computer room in DM3: The Abandoned Base in memory of the supercomputers.
 * Slipgates were designed in honor of the Apple.