The Arc II is an Autobot spaceship. Hidden under the Pacific Ocean 74 miles off the coast of California, the Arc II serves as the base of operations for a small unit of Transformers operating under the human Hawk.
The Arc II does not appear to be very deep underwater, Firewall (with Autobot assist) survived a trip up to the surface with minimal physical discomfort.
The Arc II was the command center of Hawk's effort to keep Cybertronian technology out of human hands. His contingent helping him included the human Firewall and Autobots Prowl, Sideswipe, Skids, Cosmos and Eject.
The Autobots within the base during the attack were badly damaged, but found to be repairable. The status of the Arc II itself is unknown.