The Arsenal Gear is a giant fortress that serves as the main focus in Metal Gear Solid 2: Sons of Liberty. It was created after the Tanker Incident, and it's been hiding underneath a "Cleaning Facility" known as the Big Shell.
Big Shell Incident
The Arsenal Gear is a giant fortress that wields hundred, maybe even thousand of nuclear warheads and missles. It also can manufacture a large portion of unmanned Metal Gear RAYs inside the fortress, around 25 RAYs.