ARC Carrier is the second level of The Ancient Gods, Part One, the first expansion for Doom Eternal. The level is set on the ARC Carrier, the ARC's mobile base of operations. ARC Carrier doesn't function as a conventional level, and more like The Fortress of Doom, a hub area intended to bridge levels together, although there is nothing to interact with here.
The Armored Response Coalition has used the ARC Carrier as a mobile base since the beginning of Earth's invasion from Hell. The Carrier was designed by Dr. Samuel Hayden himself after worldwide defense systems began to fail, and resistance leadership eventually became entirely localized aboard the ship as it traveled the globe attempting to stave off the forces of the Dark Realm. Though capable of long-range cannon bombardment and the launching of aircraft, the primary use of the ship was as a source of much-needed strategic intel for the ARC.