Sacred Ground (霊場 Reijō) is the term used to refer to the locations where gods and other supernatural beings can manifest.
Natural Sacred Grounds are extremely rare. They can be unexplored regions like deep valleys or thick rain forests or they can be temples, sanctuaries, or cathedrals designed by human hands. They have to be more than someplace hidden from human eyes where people are forbidden to enter. They have to naturally fill people with a desire to stay out. Someone could easily peek inside one, but people will never even think to do so. They will decide they will be happier not knowing if the alternative is exploring a place like that. It is only in these places the gods will appear.
Modern technology is thought by some to have robbed the gods of some of these Sacred Grounds, as anyone can use the Internet to easily search for satellite photos or freely explore 3D models of the pyramids. The truth is that if they are really one of the mystical locations spoken of in the summoning business, people will avoid digitalizing them or viewing that data, even if they have the technical capabilities. What really happened is that technological advances revealed just how few true sacred grounds there are in the first place. There are a fair number of partial Sacred Grounds, but in those, the "exit" gets clogged up and it turns into a haunted spot, causing ghosts and monsters like the Kuchisake-Onna to show up.
Artificial Sacred GroundEdit
The Incense Grenades used in the Blood-Sign summoning ceremony create an Artificial Sacred Ground (人工霊場 Jinkō Reijō) that allows the summoning of a Material. This Artificial Sacred Ground lasts ten minutes before disappearing.