Sargasso

Sargasso (船の墓場(サルガッソー)) are various locations found in the seas and oceans of Toaru Majutsu no Index, where sea wreckages naturally gather in areas after certain conditions are met to form an artificial island. Introduced in Shinyaku Toaru Majutsu no Index, they are primarily used by GREMLIN as their bases.

Principles
The term is used by magicians, and is unrelated to around of the Gulf of Mexico. It referred to the mountain of sea wreckage that naturally gathered in areas of sea which met certain conditions: several currents striking each other, flotsam and driftwood gathering together like in the drain of a sink, and a reef or something else that caught at and allowed that flotsam to accumulate. The remains of many different ships, those that did not completely sink to the ocean floor, would gather there: fishing boats, transport ships, passenger ships, one-man kayaks, and military ships. They would be swept more than 100 kilometers over the ocean, gather in this one spot, and pile up into a mountain.

Apparently, there were countless numbers of Sargassos in the world. Although despite having no official record of their numbers, they continued to be feared as legends by the local fishermen.

GREMLIN's Sargasso
GREMLIN has two Sargasso bases, one is found in the North Sea near Iceland, and the other, being their main base, can be found near Tokyo Bay, which they somehow "placed" there. GREMLIN chose Sargasso as their base because it was a good magical site due to the ley lines, it at allowed them to remain hidden from their enemies, such as the Anglican Church, and it was an easy location to construct a fortress. Moreover, as they wanted to create Gungnir, Sargasso was chosen as it allowed them to to borrow the power of science, using the various machinery left in the ships in the Sargasson, they used it for the installation and use of a large-scale and a high-speed parallel processor. Indeed, with the help of Kihara Kagun, they were able to make a small supercomputer. Moreover, because of the many ships that make up Sargasso it can make some space inside. This is the case with the Sargasso in Tokyo, where Touma and Othinus temporarily stay inside the structure's complex labyrinth to discuss their plans.

GREMLIN made use of the various ships in the Sargasso for varying purposes, one of which is a passenger ship reused for the purpose of their living space, as well as a pool, which was used to pour gold into it for the final phases of the creation of the Lance of Gungnir. The Sargasso in Tokyo Bay was protected by Mokkerkalfe and Muspells, while the Sargasso in the North Sea was left in the protection of Loki. The latter is later destroyed by Loki during the attack by the Anti-GREMLIN Alliance.