Dive

A Dive (daibu) is the term used to refer to the act of entering to the Soulspace of a Reyvateil, more commonly known as the Cosmosphere. This is through the usage of a special device called the Dive Machine, which allows to scan the Static H-Wave bands of the Reyvateil and her partner, called Diver under this context, to send the Diver into a world created by the feelings and memories of that particular Reyvateil.

Most Dive Machines are installed in Dive Shops found at most cities and towns, so all the Diver-Reyvateil couples go to one whenever they want to try Diving. This act is considered as very intimate and personal because the Reyvateil is allowing the Diver to probe into memories, feelings and secrets she would never reveal to anyone else, which requires a great amount of courage and trust on her part. However, the Diver also has to agree to no reveal anything he or she saw inside the Cosmosphere out of respect for the privacy of the Reyvateil: not even the Reyvateil herself can be told about what the Diver saw.

What Happens On a Dive
Once the Reyvateil and the Diver have been connected to the Dive Machine and the Dive has started, the machine will first scan the Reyvateil nature of the Reyvateil and use it to find her address in the SH Server. Upon doing this, the machine will perform a different scan on the Diver to then allow the direct interaction of the H-Waves of the Diver and the Reyvateil.

On the other hand, the Diver will be sent into the Cosmosphere: a virtual world that takes form based on the feelings and memories of the Reyvateil into which he or her is Diving, and within which he or her can trigger events, meet people the Reyvateil knows but acting under the preconceptions and stereotypes she imagines them to be, and help the Reyvateil craft Song Magic. During this, the Divers on ATDs and HDs are watched closely by entities known as Mind Guardians, which make sure the Diver doesn't do anything harmful to the Reyvateil, and in occasion, they also try to help the Diver by offering them tips and suggestions about how to take on a particular problem. IPDs don't have Mind Guardians, so this make the Dives into them more lonesome and dangerous, unless there was an external entity that acted as a Mind Guardian of sorts in the Cosmosphere of the one the Diver is Diving into.

The climax of a Dive is marked when the Diver causes an event in the Cosmosphere that causes the H-Waves of the Reyvateil to peak in a positive way towards him or her, causing an event called a Paradigm Shift. This elevates the H-Wave frequency of the Reyvateil and grants the Diver entrance into a deeper level of her mind. The higher the frequency, the deeper the level. Likewise, deeper levels allow to learn more intimate and personal facts about the Reyvateil, as well as allowing the crafting of more powerful Song Magic due to the depth on the mind at which the feelings weaving it are generated. However, this also makes these levels much more dangerous due to the H-Waves of both the Diver and the Reyvateil becoming thinner and growing closer to each other, so the Diver will have to be cautious the more she or he proceeds.

On the meanwhile, the Dive Shop operator will be closely monitoring the H-Waves of the Diver and Reyvateil, so as to avoid any dangerous events from happening, and conduct an emergency suspension if anything happens, taking back the Reyvateil and the Diver safe and sound back to the real world. However, they can't seen anything that takes place on the Cosmosphere both due to privacy reasons, and because the events can't be projected because for doing that, it would be necessary to have the technology one would need to project dreams on a screen.

Dangers of a Dive
However, the Diver has to be careful, as doing a Dive poorly could cause alterations on the Reyvateil's Static H-Wave band that could severely damage her mind or even kill her. However, this would be an incredibly idiotic thing to do, as the Diver would get involved as well in the destruction of the Cosmosphere and end up with his or her mind as damaged as the Reyvateil herself due to their H-Waves being so close during the Dive that they end up influencing each other.

Likewise, while a Reyvateil is normally unaware of the events that take place on a Dive due to being pretty much asleep during it and the Dive being like a dream for her, there are a few that can train to put themselves in a state akin to that of lucid dreaming and get full control of everything that happens in the Cosmosphere. There have been some malicious Reyvateils that even lured Divers to Dive into them to end up trapping them into an inescapable empty space or outright killing them in the Cosmosphere, causing an intentional Critical Down which ended leaving the Divers brain-dead and turned permanently into vegetables.

And while there are times in which a Critical Down has been accidentally triggered, this normally is due to the Diver trying to access a Cosmosphere without the permission of the Reyvateil (as she can agree to doing it superficially but be against it subconsciously) or triggering an event in a way that is inadmissible for the Reyvateil herself. When this happens, the synchronization between their H-Waves gets broken in a completely irreconcilable way and the Diver gets forcefully ejected from the Cosmosphere, which could potentially kill him or her.

Laws
Not much is known about the legislation in Ar Ciel or each of its regions about the laws governing the act of Diving, but there is one known: unless it has been proven that the Reyvateil intentionally caused the death of the Diver when a lethal Critical Down occurs by messing with her Cosmosphere, the death of the Diver will be assumed as an accident and the Reyvateil won't be prosecuted due to it. Therefore, all Reyvateils have some safety in case they were coerced into a Dive and accidentally killed the Diver due to subconsciously rejecting their entrance into their minds.

Limits
Not everyone can be a Diver: all humans and Halfblood Reyvateils are able to do so because they possess human natures. However, the Pureblood Reyvateils lack human natures, and this makes them unable to be Divers. Likewise, it's impossible for anyone to perform a self-Dive due to the machines having their pod interfaces separated into Diver and Reyvateil, and there isn't a single machine that has both interfaces coupled in the same pod.

Similarly, while it's possible to Dive into all Reyvateils, the Origins have some peculiarities that will be explained below: Additionally, the maximum level into which Reyvateils can be Dived into is Lv. 9. This is due to Reyvateils having roughly the same spiritual frequency band as the humans, and the minds for both end at an H-Wave frequency that equals Lv. 9. Therefore, there is nothing else to be seen at that point.
 * Shurelia and Tyria are Diveable, but since they don't possess a Cosmosphere due to having a HD-Cellophane as the basis for their minds, trying to Dive into them ends leading the Diver into their respective zones of the Binary Field.
 * Frelia does have a Cosmosphere, but no Dive Machine (excepting possibly the ancient VR-21 machine) is capable of accessing it due her personal SH Server being different from the Beta-6D Servers used by all the other Reyvateils. Therefore, Divers get redirected to her zone of the Binary Field.

As for humans, it's impossible to Dive into them because they don't have a connection to a SH Server due to having an inactive Reyvateil nature. Due to this, they neither have a Cosmosphere or Soulspace Address the Dive Machine can scan to make the virtual world, and their minds are structured in such a complex way that the Dive Machine can't make sense of it. Reyvateil can because their minds have a much simpler structure, which even the Halfblood Reyvateils could replicate naturally thanks to the activation of their Reyvateil natures.

Non-Conventional Dives
Aside of the typical Dives into a Reyvateil, there were a few special cases in the series:
 * One was Tyria Diving into Finnel to find Ar Ru. While normally Pureblood Reyvateils can't do it due to lacking human natures, in this case it was possible due to using the VR-21, which is pretty much the prototype to all current Dive Machines.
 * Other was the Dive into Ar Ciel by using the XP Shell. In this case, the Shell contained equipment capable of parsing the ultra-high frequencies used by the Wills of the Planet, as well as Tyria herself being capable of understanding Ar Ciel due to having absorbed Ar Ru with EXEC_COSMOFLIPS/. Likewise, while it seemed like everyone in the party was Diving, actually Tyria was the only doing it: she Dived, took the H-Waves Ar Ciel was exchanging with her and sent them to Harvestasha XP to get them translated into a form the others would be able to understand.
 * Finally, it was the usage of the Dive Machines in the Hot Spot to Dive into Metafalica's Soulspace. While not as advanced as VR-21, the Dive Machines from the Hot Spot are still more advanced than the conventional ones found at the Dive Shop, which also allowed this type of Dive.