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), 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.

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 death, 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.