The Batsub was an underwater vessel used by Batman.
When Earthmover kidnapped Jackie Wallace and her foster father, Bill, Batman took the Batsub to find his underwater cave. The sub was equipped to get there by going through Earthmover's forces. As Batman fought off Earthmover's dirt monsters, he got Jackie and Bill to the sub safely. He then hurriedly got them out before a cave-in occurred and tried to use the sub's torpedoes to blow a boulder blocking their way, but to no avail. Ultimately, Earthmover used his last bit of strength to remove the boulder and let Jackie go. Batman, Bill, and Jackie all managed to leave safely in the sub.