Leviathans were immense generation starships designed by humanity in the mid-22nd century, each containing a functioning ecosystem on board so that the early colonists (who would never reach the new world) would have a pleasant life. These were the only way of getting enough humans beyond the solar system to properly colonise other worlds - but by the time the first prototypes were created, faster-than-light travel was discovered and the Leviathans became obsolete.
The Sentinels of the New Dawn placed themselves in suspended animation on one Leviathan which they populated with clones and set on a course towards the planet Phlegethon. Centuries later, the ship was visited by the Sixth Doctor and Peri Brown. (AUDIO: Leviathan)