Underhill is a term referring to a sort of magical sub-reality associated with the island of Yuriba. Heavily infused with magical essence, the Underhill area appears to be generally inaccessible to normal humans. Verbal accounts from those who have visited or lived there indicate that the Underhill realm is highly thought-morphic except in certain controlled sub areas, that in most areas it maintains a pleasant, balmy climate, and that it seems to only very loosely correlate to the features of the material landscape.
Natives typically refer to Underhill as being subdivided into pockets; these being areas that have had their morphology primarily stabilized by consistent inhabitation and application of will. These pockets vary widely in size, from tiny homes to wide personal estates to one extremely large pocket that holds the majority of Underhill civilization. Exactly what lies between these pockets is unclear, although the ending of a pocket can usually be seen as the landscape dissolves away into heavy mist. For most residents of the village, the best known pocket is the one that contains the Underhill Tree Estates apartments. Others that have been encountered are a private estate that breisleach says is a retreat for priestesses of the Lady and an always wintry pocket discovered by Lumi that contains the remnant of an ancient, possibly Lilian, temple to Fuyuzora. More recently, some longer term immigrants have been taken to a pocket where the primary temple of Harumeku is located, likely in conjunction with native true name rituals.
In addition to these pockets there are also a few areas where Underhill's properties bleed over and blend into those of what is variously known as the material plane, Overhill, over the hill, etc., this being the plane wherein the main village lies. A good example of such an instance is the Faerie Forest; there are hints that many of the Jusenkyou Springs lie in such dimensionally blended areas as well.
The known native populations that live in the Yuriban Underhill include kitsune and fae. Presumably, whatever buildings or edifices they have created Underhill do not effect the material reality, since there seems to be no evidence of them, and the locals claim that the few ruins that can be found on the island are from now extinct cultures that existed during the Ancient Era.
There is at least one known, but hidden, access point directly into a portion of Underhill in the material Yuriba.