Prime_Evil said:
Did you take the Red Pill or the Blue pill this morning?
There's a serious theory amongst cosmologists that the universe requires conscious observers to force it into a definite state. Until conscious observers arise, all of the possible histories of the universe exist in a state of superposition. By the act of observing the universe, conscious minds collapse the probabilistic waveform and force it to assume one state or another.
"With our thoughts, we make the world." - The Tathagata Buddha.
The best way of revealing extraplanar stuff is to introduce it a piece at a time. Artefacts from other places, which should not be - describe the contents of a vial as "waters from some fabled river in the Underworld in which, according to the legend, the departing souls of the dead must bathe in order to wash away the burdens and stains of their past lives before entering the afterlife, cleansed," rather than "Oh, it's River Lethe water. Drink it and it gives you permanent amnesia."
Or perhaps some fisherman dredges up an ancient mechanism from the bottom of the sea that looks like part of some larger mechanism, all brass and gears and cogwheels; and the characters end up learning that it formed part of a strange realm behind the worlds, an Orrery behind The Curtain, where each globe is a realm unto itself; strange beings from a plane where the conscious minds there correspond to the Jungian Archetype of the Shadow here.
And then gradually, introduce them to the realms - at first by accident; a miscast Teleport spell throws them into a mirror realm where the people they know are all evil, twisted reflections of their normal characters for example, or a quest takes them through a doorway into an underground realm where they travel along a dark river to a place where dead souls dwell ... or did they really visit there in person, or did hallucinogenic fumes overwhelm their bodies while they were trapped down there?
At some point, the characters discover spells, meditation techniques or devices which will allow them controlled access to those otherplanar worlds - a magic belt, a doorway, a ring, a vial of potion - but even then, those sojourns should be risky propositions.