Originally intended to take the Decepticons back to Cybertron, the Decepticon space cruiser served as their base of operations on Earth. The space cruiser carried many weapons, including a huge cannon, and its hull was able to withstand the crushing water pressure of an ocean — which is exactly where it ended up.
The Transformers cartoon
After coming back online and leaving the Autobot's vessel, the Decepticons quickly made efforts to plunder Earth's resources and return to Cybertron. Using the same design as their original vessel, the Decepticons began acquiring materials to build a new ship and go home.
After several days (months?) of amassing Energon cubes, fighting the Autobots, and destroying a few buildings for raw materials (or sport, depending on who did the destruction), the Decepticons were ready to leave. The Autobots made one last desperate assault on the Decepticon camp, but only managed to stall their delay. Optimus Prime made the bold decision to chase after them using Sideswipe's jet pack, but the weapons on the space cruiser shot him down.
However, Mirage managed to sneak aboard, and took advantage of Starscream's latest coup attempt against Megatron to destroy several pieces of equipment. Mirage managed to escape, and the space cruiser crashed into the ocean. The Autobots believed the Decepticons were destroyed, but they managed to survive the crash.
After several months on the ocean floor, the Decepticons built several nearby buildings with a telescoping docking tower, a ship-to-surface "elevator" allowing them to enter and leave at will without getting wet, establishing it as their primary base on Earth.
The vessel's fate after the Decepticons conquered Cybertron and abandoned Earth is unknown.
- See also: Decepticon Headquarters