The Ultrasub was a submarine designed and built by Dr. Ricer at a dock right next to his house. Ricer had been planning to take a trip to the bottom of the ocean for 20 years.
Prime Evil ordered Apparitia and Long John Scarechrome to steal the Ultrasub to find a lost decoding box. Their search led straight to the old sunken pirate ship Lucky Lady.
However, Ricer's daughter Amanda snuck on board to protect her father's invention and the President of the United States personally called the Ghostbusters in to retrieve the sub.
The Ultrasub makes her only appearance in the episode Pretend Friends.