The Ark was the name given by Dodo Chaplet to an enormous spacecraft built circa 10,000,000 to carry the majority of the human, Monoid, animal, and plant population of Earth before it plunged into the Sun. (TV: The Ark)
The ship took 500 years to construct and held a city, a jungle, a desert, cultured land, and polar regions. (PROSE: The Ark)
Operated by the Guardians and Monoids, with all others miniaturised and placed in suspended animation, the ship would journey 700 years to Refusis II. The ship's loading bay held Launchers for landing on the planet.
The First Doctor, Steven Taylor and Dodo visited the Ark twice, at the beginning and end of its journey. (TV: The Ark)