A place transposed over the world the average mortal interacts with it the super-space of the Faeries called the Sunlit Grove. Exclusively for Fairy-kind and their guests the Sunlit Grove cannot be penetrated by any being under the strength of the most powerful dragons or genies. The Fairy world is a part of the fairy-kind heritage, and any being that can trace its lineage to their loins will find another realm that was designed for their Fae Ancestor, grown from them like any human has an imagination, only if it could be made 'real'. How real the Sunlit Grove is is subjective, the nature equally so, and both are dependent on the Faerie and their mood. In their miniature realm they wield godly power.
Fairy realms are simultaneous in time and space and can be massive or atomic in size depending on the fairy breed/individual who spawned it, using the open doors and winding paths throughout the realms can get a mortal from one pocket realm to another without fae help, but doing so leaves the traveler exposed to the elements of the kingdom, without a fairy anchor to keep some portion of the world solid it can melt away underfoot and depositing the traveler in any new place imaged by the nearest or most influential fae.
With the possibility that the air in your lungs could turn to living fire by accidentally crossing into another fae's realm most take a guide. With the help of a guide the traveler can cross a great distance by jumping through these fae-holes.
Faeries were once rampant throughout the world, but as other creatures discovered their magic properties most were subsumed, outright exterminated or hunted. While some can be killed as easily as any mortal the soul of a fairy is forever a part of it's home realm, be it shared or solo this home realm is a part of the fae who lives there like an arm or the ability to heal from wounds. Often times a doorway to this realm can be representative of the actual nature of the realm beyond, a water sprite's home appearing as a pond or fountain in mortal realms or a palace with many rooms that are all pocket realms for other fae.
Whatever the doorway the passage is frequently not instantaneous and a tunnel, regardless of what it's made of or what it contains all these passageways are considered tunnels. The tunnels that connect all fae realms are themselves the oldest realm. All of them connecting realms to one another, vast, and built on a structure that even other faeries do not understand. This empty but full tunnels are a largely stable place, stable but intensely powerful the tunnels are said to transform people who pass through them, shinning a light on all their secrets and blindness and revealing the honest truth of a being.
The Tunnels feel inhabited and if you have a light source it seems that their formless bodies writhe just out of sight, the sounds of their passing on the other side of the earthen walls makes it seem like they could collapse and spew forth some unseen horror. As with all things related to faeries the length, width, shape, and make are different depending on the realms they connect and the tunneler who made the way.