Door

Looking to be actual doors fashioned from materials unknown, these strange phenomena are manifestations of mana that appear around the world in places of dense population such as cities and sometimes even smaller towns. Inside these doors it is as if another world lay within created to be seemingly random and have shown to include various types of monsters and challenges that threaten to be released onto Earth if left unattended.
Dealing with these Doors and the dangers they threaten to unleash onto the populace is the job of the Divers and because of this being a job for them the various Doors are separated into ranks according to the strength of the Doors.

Registering a Door

All Doors are recorded by the Association through the use of an item that calculates the average amount of mana in the construct and the proctor registers the rank of the Door accordingly. Once it is registered, the Door is either reserved for the lower ranked Divers contracted by the Association, or in the case of a higher ranked Door they are put onto the Association's Auction for guilds and other groups to purchase the rights to reserve it for themselves. Though a minimum of ten people are needed to reserve a Door, so many people have created forums looking for parties or follow guilds that are recruiting to join them on a Dive.
This Auction can be accessed through information terminals that access the GeoNetwork at select locations maintained and monitored by the Association.

What to Expect

When a Door is encountered there are usually Association personnel already stationed to keep an eye on the construct to verify that no one wanders into them or they do not Open suddenly, but the main reason is to ward off any potential people wishing to sneak in and Close it while taking away any possible income from them. During Dives, the personnel also set up a rest station for the party that was recruited to deal with it, these rest stations usually provide free food and coffee or other refreshments to be able to accommodate early mornings and long Dives.
Diving is the term used when a party enters a Door and immerse themselves in the otherworldly space that expands out of nothingness. These spaces vary in environments dependent on how much mana was drawn in for it to form, the weaker Doors seeming to only be able to conjure reflections of spaces with a splash of the environment the monsters are from such as a E ranked Door being several floors of an office building mixed with jungle trees and vines vibrantly growing throughout the space, while a much stronger Door opens up into a landscape like an open country side.
Whatever people find on the other side they are normally able to exit the Door to make sure they are prepared to be able to properly tackle the environment and monsters, even though this information should be provided by Association members positioned as Scouts to be able to list more information on the Door for the Divers.

Manifestation

When they appear on Earth they are seen to be doors of varying sizes both in a familiar and ancient style made from alien materials. Some Doors simply look to be made of old wood, others could be made out of metal or stone. The strength of the mana when the Door forms changes its size and how it looks, the weaker ones looking to be simpler and made out of natural materials, others with more mana have been observed to be larger, made out of stranger metals and materials, then the strongest of them seem to be constructed with depictions of the beings found inside, an example of this would the Tower Bridge Door.

Localization

They are most commonly found to appear in places of population and grow even stronger the more people there are, though it is rare some Doors have strangely appeared in the middle of nowhere. The Doors appearing in the wilderness or secluded may be linked to research on leylines.

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In case a Door cannot be closed by the registered party, an investigation team is sent to determine cause of failure and whther the Door requires re-evaluation. If the ranking is found to be stable, the Association returns the Door to the Auction if the holder withdraws their reservation and inputs the risk assessment onto the description in warning to those not able to handle the Dive.

Doors are ranked from the following with F being the weakest and S being the strongest rank. Next to them are the minimum rank of Divers recommended to enter such Doors.

• F- F
• E- F
• D- E
• C- D
• B- B
• A- B
• S- S

Those of rank D and below are reserved only for the Divers that are still working under the Association, while the Doors that are ranked from C to A are normally taken care of by guilds.
Any Door found with a ranking of S is considered a national threat and dealt with by a combination of the highest ranking Divers specifically recognized by the Association.

Doors have been labeled as arcane constructs as they have been found to be made entirely out of pure mana. Because of this, any attempt to physically destroy these Doors have failed and those whom have tried to magically close them either fail or burn themselves out resulting in death.

Closing a Door

To be able to Close a Door properly, all of the monsters and the boss that is distinguished by its increased strength when compared to the other monsters found found inside are needing to be killed. Proven through many attempts at breaking the connection the boss of the Doors are what keep it stable and regardless of however many monsters are left, if the boss is killed the Door will close within a few hours at most.
One way larger guilds have dealt with the closing of Doors is that they restrain the boss, or do not engage it altogether if possible and keep a watch over it after all the other monsters are defeated so that a collection team can work on gathering corpses and other materials from the Door without worrying about it closing on them.
How a Door closes is that it begins to release the excess mana into the area as the physical representation diminishes as if fading from the world like a ghost. When this happens, the completion of the task is reported to the Association to accurately account for the number of Doors they have registered.