Dimensional Overlap

"Dimensional Overlap" is a boring, prosaic name for a terrifying phenomenon. As best as scholars have been able to determine, the reality in which Flimflam's humans live overlaps with the reality of the divine realm. Or, possibly, realms. There is significant debate on that point.
This law of reality first came to widespread human attention at the beginning of the Vault Revolutionary Age, though at the time people were completely unaware of the transformative effect their discovery would have across the world. Miners came across a lustrous, pearlescent black stone (soon to be known as Vault Stone) that was easily carved and yet sturdy enough for architectural use. Despite numerous unexplained disappearances among the workforce, mine owners eagerly brought the stone up to use as tribute to nobility or as a newly minted status symbol for the wealthy. In some instances, complete frames were created--most often by flooring a hallway with vault stone tiles interspersed with carved vault stone door frames. Though the process of creating a portal between the mundane and divine realities is now standardized through the use of Vault Crystals, the crystal itself is not a necessary part of the process. Anyone who has made the appropriate sacrifices and passes through the vault stone frame will be transported to a Vault.
As such, the disappearances that plagued the mines only increased as more and more vault stone was used in architectural design. But now that powerful individuals rather than poor miners were involved, investigators took their duties (and the stories of survivors who claimed to have seen wild, monster-plagued caverns) much more seriously. The root cause was ultimately determined to be the property that vault stone is now known for: the ability to create a portal linking the mundane reality and the divine reality, even if only for a limited amount of time. This swiftly became vault stone's primary property. (and led to a second major wave of redecoration in wealthy homes and public buildings) Scholars of myth and fairy tale also suggest that a number of traditional tales can be explained with reference to Dimensional Overlap--consider, for instance, if lighting Hani's Fabulous Lamp is really a more likely trigger for adventure in a fantastical, treasure-filled realm than accidentally walking through a vault stone tunnel.
The abrupt introduction of tangible proof of the divine (see below, "Manifestation") ignited chaos across Flimflam. As might be expected, this was particularly pronounced among regions which worshipped deities whose touch was not found (at least, not yet) within the vaults. The Proving, as it came to be called, completely reshaped the world's political and cultural maps in a little under two years. The full effects on industry are likely still to come.

Manifestation

It is theorized that the overlapping realities are metaphysically moving in relation to each other, hence why vault portals only remain open for limited (and changeable) amounts of time. No reliable method has yet been determined for returning to a particular vault area, despite significant investment among those sacriligeous entrepreneurs who want to mine the divine realm.
The question of how we know that the vault reality is, in fact, divine is one that smug, yet sheltered, scholasticians often ask in ill-fated attempts to look intelligent. The answer is immediately obvious to any who have been inside a vault. They abound with shrines to the divine beings as well as religious iconography, such as bells for Alea, Goddess of Chance. Some Vault Hunters claim to have personally met with the Gods and Goddesses or received their especial blessing. Though no scholar has yet recorded yet an experience, several blessings have been tested empirically, and the balance of evidence tends towards agreement with the adventurers' tales.

Localization

The most obvious and well-known manifestation of Dimensional Overlap are Vaults, or, more precisely, the portals which Vault Hunters use to enter Vaults. Each portal is a physical space in which the metaphysical strands of mundane reality and divine "reality" are woven together, allowing transit between the two planes. Portals can seemingly be created anywhere, presuming that one has access to enough Vault Stone to create a human-sized frame.
Much ink (and, occasionally, blood) has been spilled on the subject of why vault stone causes Dimensional Overlap to manifest. The leading camps are the Borrowers (who argue that vault stone is a material originally from the divine dimension), the Invokers (who argue that vault stone creates a connection to the divine much like prayer or sacrifice), and the Transdimensional Boundary Effect Theorists aka "Bounders" (who argue that vault stone is created through the metaphysical friction of two realities rubbing together). Each camp has some evidence on its side, such as the existence of vault stone within the Vaults as well as within mundane reality, as well as key questions that proponents cannot yet answer convincingly (at least, to an unbiased observer).

Type

Metaphysical, Divine