1 Villager: Here's the teleport gate. {shows the adventurers the teleport gate, which glows softly in lurid colours}
2 Mordekai: How does it work?
3 Villager: Magic.
4 Mordekai: That's such a cop-out.
4 Kyros: Not when it's true.

A wizard did it.

Literally.

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2019-12-29 Rerun commentary: A video of the teleport gate would be interesting. Shifting patterns and changing colours, with a cool glow. Unfortunately this is a static medium, so you'll just have to imagine it.

We sort of have a standardised look for teleport gates in visual media. As with most things, presumably when we invent real teleport gates, they'll look totally different.