Slipgate

A slipgate is a mechanical/computerized means of travelling between Quake's many dimensions. They're most commonly found in Base themed levels and usually consist of a large humming and glowing machine (pictured) which leads to a destination pad (the floor pad in front of the big machine) in an alternate dimension.

Quake lore is far from conclusive, but it is often mooted that the slipgates are a human invention which first allowed Quake's forces to enter the human world and vice versa.

Trivia

According to John Romero the design "was designed in honor of the Apple ][. Specifically, the ] and [ ....a Slipgate side view.."