Vanishing Tunnel

The Vanishing Tunnel (カキキエ隧道), also known as Academy City's Greatest Taboo (学園都市最大の禁忌) or the Taboo (禁忌), is an underground space beneath Academy City's School District 10, a mass of AIM Diffusion Fields resulting from a failed experiment involving the Imaginary Number District.

Principles
The Vanishing Tunnel is not a conventionally constructed tunnel, being a giant mass of AIM Diffusion Fields which manifested in the physical realm as an underground space, over 250 meters across and perhaps more than twice as deep, as the result of a failed project to extract material resources from the Imaginary Number District. Although the Vanishing Tunnel is described as a temporary space, one which is said to not truly exist, it apparently has a natural half  of around 12,000 years.

The project's goal was to cut out a portion of the Imaginary Number District, part of the "city" created by the aggregation of AIM Diffusion Fields, and extract inorganic materials and resources from it. The extraction would have been done on a macro scale rather the microscopic work performed in the particle accelerator and apparently cost very little. The researchers guiding the power through the city ultimately failed to control it, did not predict the form it would end up taking and were unable to erase the underground space which had been created. In referring to the incident, Kihara Hasuu mentioned that in the worse case, the whole Imaginary Number District could have been converted into real numbers and crushed Academy City, but thought that they might have found some success if they had cut off a piece at a workable size and found a way to control it.

The project's major failure and the tunnel it created were covered up, with even members of the Academy City Board of Directors (such as Valart Signal, in charge of construction and deeply involved in the Dark Side) not fully aware of the Vanishing Tunnel's true nature. With a nonexistent pathway unofficially connecting Academy City to the outside world, there was a risk of Academy City's information and technology leaking through it, with certain people (such as Kihara Hasuu) seeing a risk of the outside world collapsing from cultural exchange and technological contamination. There was also the possibility of the outside world being distorted by entities coming from within the Imaginary Number District.

The Vanishing Tunnel acts as a loophole (抜け穴) and line for the Dark Side of Academy City, which consumes many times more than the official amount of material resources that Academy City imports from the outside world. These extra materials are secretly taken into the city through the tunnel by train, with recycling efforts being promoted on the surface partly to conceal the discrepancy.

Being an aggregation of weak powers, the Vanishing Tunnel is the opposite of an entity sustained by a powerful energy emission, such as the artificial ghost Frillsand G. As a result, the two of them cannot co-exist in exactly the same place. Kihara Hasuu exploited this to neutralize Frillsand by forcibly bringing her together and into conflict with the tunnel, causing the ghost to malfunction. According to Hasuu, although the odds were low, if the tunnel had lost, the temporary space would have vanished and those inside been buried. After the original high energy emission ceased, the collection of weak powers were able to act as a sustaining substitute, focused towards a single point.

Operation Handcuffs Arc
On December 25th, during the events surrounding Operation Handcuffs, a large number of people from the Dark Side of Academy City, having found out about 'Academy City's Greatest Taboo', headed for the Vanishing Tunnel in order to escape from the city.

The majority of the people who neared the tunnel were killed by Drencher Kihara Repatri's artificial ghost Frillsand, however Kihara Hasuu managed to neutralize and trap Frillsand by using the properties of the Vanishing Tunnel against her. A battle was subsequently fought in the tunnel between Hasuu's android Ladybird and Hamazura Shiage, who was trying to help the Child Errors in Drencher's care escape with support from Aneri.

After Hasuu's, Hamazura chose not to escape through the tunnel, returning to the surface. When Frillsand escaped from the trap and witnessed Drencher's corpse, the tunnel's space gathered in towards her and its collection of weak energy was absorbed in place of the original high energy emission, as the artificial ghost cried out and grew into a vengeful spirit.

Post-Handcuffs Arc
By December 29th, Frillsand's control over the Vanishing Tunnel had been usurped by the ghost of Kihara Hasuu, who had used a reverse-engineered method to survive as the same type of artificial ghost that she was, and had managed to capture and ensnare Risako. Still unstable and not in total control of himself or the Taboo, he attempted to manipulate the girl while making plans to safely escape and survive outside the Vanishing Tunnel in such a way he could continue his research, while Frillsand was unable to reach and save Risako due to his manipulations.

Having learnt of Risako's situation from Frillsand's appearance during the Overhunting prisoner escape and a phone call from Accelerator, Kamijou Touma headed for the Vanishing Tunnel accompanied by Alice Anotherbible and Shirai Kuroko, though only he was able to get through the spatial distortion to face Hasuu. A critically injured Hamazura Shiage also headed for the tunnel, having awoken in the ICU to a cry for help. The two boys fought the evil ghost in control of the Taboo,   aided by Sodate, Kihara Noukan, and then a repaired Ladybird. Eventually Qliphah Puzzle 545 managed to free Risako and draw out the desires necessary for her to reject Hasuu's influence, resulting in the ghost being ejected from the Vanishing Tunnel into the Imaginary Number District, where Kazakiri Hyouka finished him for good.

Trivia

 * In Toaru Kagaku no Mental Out, a clash between the two Relics causes a spatial warp and produces glimpses of scenery which Nogi Yasuhito confirmed to be of the Imaginary Number District.  Among the two is a circular space, somewhat reminiscent of descriptions of the Vanishing Tunnel. Nogi has acknowledged that the Vanishing Tunnel is like that, but claims not to know if the circular space shown in this case is the actual one or something similar.