Aqua Tunnel

The Aqua Tunnel is a type of submarine, supposedly providing luxurious trips round the world, but actually a cover for Necessarius's marine prisons.

Principles
To the outside world, the Aqua Tunnel is a glass submarine made out of a thick bulletproof resin, which is meant to produce the same effect as that of the clear tunnels found in aquariums. The Aqua Tunnel carries passengers on a luxury tour of the world's oceans, however the race for tickets is harsh and no normal person could usually hope to get one.

However the Aqua Tunnel is actually a cover for the marine prisons which are used by Necessarius to transport captured magical criminals from around the world, which travel through the seven seas at set intervals. They were originally created to prevent magicians from escaping before being brought back to England. As it would be too costly to fool the Science Side's radar and sonar, instead a situation was created where it would seem as though there were 12 ships giving trips round the world, though in reality 11 of them were prisons with 1 actual luxury submarine as a decoy to avert suspicion.

The marine prisons lack a control room to prevent prisoners from hijacking the ship. Instead, they are controlled remotely by seven spiritual items known as the Cape of Good Hope, one for each of the seven seas.

Toaru Majutsu no Index SS: Kanzaki
One of the marine prisons was taken over by the prisoners aboard it, who used a spell based on Naglfar to seize control of the ship from the Cape of Good Hope, intending to run the ship aground in India and escape. In order to prevent this, Necessarius dispatched Kanzaki Kaori with orders to kill everyone aboard, jailers or prisoners. However Kanzaki wasn't willing to go through with those orders and tried to find another way to resolve the situation. After defeating another magician sent to fulfill the original orders, Kanzaki challenged the prisoners onboard the submarine, who had witnessed the battle. In response, they surrendered their control of the submarine and it resumed its course for England.