Dive (ダイブ, daibu?), also called Mental Endoscopic Surgery (心の内視鏡手術, kokoro no naishikyoushujutsu?) at one point, 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 done 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 anything 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 and deploy their feelings and the Reyvateil's through the ω-Spectrum, to then allow direct interaction between the H-Waves of the Diver and the Reyvateil. Then, the Dive Machine will automatically tune the H-Waves of both the Reyvateil and the Diver in order to find the frequency level to which the Diver is allowed to access by the Reyvateil at that point in time. Note that this means that each time a Diver enters a Reyvateil's Cosmosphere for the first time, they will have to start from Lv. 1 even if said Reyvateil has reached a higher level with another person.
Once this process is completed, 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 makes 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 IPD the Diver is Diving into.
The climax of a Dive is marked when the Diver triggers an event in the Cosmosphere that causes the H-Waves of the Reyvateil to peak in a positive way towards him or her, causing a phenomenon called the 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 see anything that takes place on the Cosmosphere in part due to privacy reasons, and in part because the Dive Machines don't have the technology necessary for what would essentially be projecting dreams into a screen: in other words, receiving and analyzing the signals sent by the Reyvateil and the Diver to then convert them into data. Therefore, they can only observe the H-Wave clashes and energy changes between wave frequency bands that take place between both parties involved in the Dive.
Dangers of a Dive
The Diver has to be careful though, 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. 
The worst case scenario for a Diver would be cerebral death in their case due to their H-Waves being devoured by the Reyvateil's H-Waves, while the rest of their body would remain unaffected due to the Dive Machine filtering out sensations like those gotten from explosions or being choked.
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.
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. However, if it can proven that the Reyvateil intentionally tampered with her Cosmosphere with the purpose of expressly killing the Diver, she will be apprehended and tried for murder, though such cases are a rarity.
Additionally, while the Dive Machines can be artificially tuned by the Dive Shop operators to give a Diver access to Cosmosphere levels to which he or her hasn't gained access, they would instantly get killed upon entering the Cosmosphere from the Critical Down they would receive due to the large frequency difference between their minds and the higher level. Therefore, performing this sort of tuning is completely forbidden to all Dive Shops in Ar Ciel.
Diving Under Exceptional Conditions
It's still possible to Dive within a Reyvateil while the Towers are in a suspended state because the SH_RAM is still working as normal, so access to the Soulspace is possible under these conditions.
However, accessing the Soulspace if the Reyvateil is outside of her SH Server's range is impossible: the Diver may attempt to do it, but the only thing they will get will be getting sent into a completely black or white void, which in data terms could be represented as 00 due to being impossible to access the information stored in the SH_RAM. The only reason why the Diver is still allowed to do this is because the Install Port works as a secondary access point for the Soulspace, but even so, they might be able to gain a small or blurry glimpse of the Reyvateil's world if they manage to enter a bit into the SH Server's range during the Dive.
Minimum Ages for the Dives
Reyvateils must at least be of 13 years in age before they can be Dived into, as their Soulspace doesn't get structured as an actual world until that age, and trying to enter it before time would just lead the Diver into a chaotic vortex of feelings, energy and sensations, which would have a great probability of killing off both the Reyvateil and the Dive.
As for the Divers, since allowing children to Dive into difficult and harsh worlds like the Cosmospheres might be would be a great cruelty with bad results as the most likely outcome, only people older than 15 are allowed to Dive into a Reyvateil.
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. This is the same reason why a Reyvateil acting as a Diver can't easily craft Song Magic if she is Diving into another Reyvateil: the only way in which they would craft something would be if they had a major shock or epiphany that would lead them to craft Song Magic in the real world.
Similarly, while it's possible to Dive into all Reyvateils, the Origins have some peculiarities that will be explained below:
- 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 β-6D Servers used by all the other Reyvateils. Therefore, Divers get redirected to her zone of the Binary Field.
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 to the location for Lv. 9. Therefore, there is nothing else to be seen at that point.
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 nor 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 due to theoretically having them composed by other wave types aside of H-Waves. Reyvateils 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.
As for how long a Diver or Halfblood Reyvateil may remain in a Dive Machine, the lawful limit is up to three hours, as despite Dives being similar to dreams due to there being a disconnect in the passage of time between the real world and the Cosmospheres due to D-Waves and H-Waves moving at differing paces in different dimensions, as well as the omission of unimportant events; if they stay for too long, they could end up suffering from malnutrition with the consequences this implies: they would start getting lethargic, would end up falling into a coma and finally would die from starvation. And even if the Dive Machine was equipped with life support systems to keep them from dying, they would suffer from a condition called Bed-Peeing Syndrome, in which all the movements and actions they took within the Soulspace would be imitated by their real-world body.
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.
- ↑ Ar Portal - Ar Ciel Technical Data Compilation Room Issue #8: "Singing Hill" Villager Recording Successfully Completed!
- ↑ Ar Portal - Toukousphere Issue #69, Technical Service Center
- ↑ Ar Portal - Toukousphere Issue #32, Technical Service Center
- ↑ 4.0 4.1 Ar Portal - Ar Ciel Technical Data Compilation Room Issue #5: The Post Count Has Hardly Changed
- ↑ 5.0 5.1 5.2 5.3 Ar tonelico: Melody of Elemia Setting Encyclopedia - Wave Science Lecture, page 115
- ↑ Ar Portal - Toukousphere Issue #60, Technical Service Center
- ↑ Ar Portal - Ar Ciel Technical Data Compilation Room Issue #13: Congratulations on the Release of Hymmnos Musical Cocona!
- ↑ 8.0 8.1 Ar Portal - Ar Ciel Technical Data Compilation Room Issue #10: About the Singing Hill CD
- ↑ 9.0 9.1 Ar Portal - Toukousphere Issue #70, Technical Service Center
- ↑ Ar Portal - Toukousphere Issue #53, Technical Service Center
- ↑ Ar Portal - Toukousphere Issue #84, Technical Service Center
- ↑ Ar Portal - Toukousphere Issue #78, Technical Service Center
- ↑ Ar Portal - Toukousphere Issue #23, Technical Service Center
- ↑ Ar Portal - Toukousphere Issue #76, Technical Service Center
- ↑ Ar Portal - Toukousphere Issue #33, Technical Service Center
- ↑ Ar Portal - Toukousphere Issue #57, Technical Service Center
- ↑ Ar Portal - Toukousphere Issue #37, Technical Service Center
- ↑ Ar Portal - Toukousphere Issue #98, Technical Service Center
- ↑ Ar Portal - Toukousphere Issue #50, Technical Service Center
- ↑ Ar Portal - Toukousphere Issue #77, Technical Service Center