The gigantic mobile fortress hidden beneath Big Shell in Metal Gear Solid 2: Sons of Liberty is Arsenal Gear.
Arsenal Gear is a massive submarine-like platform designed to house the AI system GW and support the Patriots’ control of information. Its existence explains why Big Shell was constructed and why the facility’s apparent environmental purpose conceals a military operation.
The fortress becomes the central location of the game’s final act. Raiden discovers that the terrorist crisis, the simulation-like structure of his mission, and the manipulation of digital information are connected to Arsenal Gear’s systems.
Arsenal Gear is distinct from Metal Gear REX and Metal Gear RAY. Those are humanoid weapons, while Arsenal Gear is a large mobile command and information platform. Outer Haven, meanwhile, belongs to the later events of Metal Gear Solid 4.