Necessarius Underground Labyrinth (ネセサリウスの地下鉄迷宮 Nesesariusu no Chikatetsu Meikyū?) refers to the complex arrangement of underground tunnels found in London used and maintained by Necessarius. It was used by Necessarius to test new recruits before becoming official members, notably the Amakusa Christians who passed the actual physical test instead of the "virtual" one that was prepared for them.