Ark

From TARDIS Index File, the free Doctor Who reference.

Ark
Type:	Generation starship
Created by:	Earth, 10,000,000
Appearances:	DW: The Ark

The Ark was an enormous spacecraft built to carry the entire Human, Monoid, animal and plant population of Earth, which was theatening to fall into the Sun, to a new home on Refusis II. Since the journey would take 700 years, all but a few were miniaturized and placed in suspended animation.

The Doctor, Steven Taylor and Dodo Chaplet visited the Ark twice, once at the beginning and once at the end of its journey. It was Dodo who gave the ship the nickname of the Ark, as humanity had long since forgotten the story of Noah. (DW: The Ark)

In fact, Earth was preserved by the National Trust and reverted back into a "classic Earth" before finally being destroyed by the Sun in the year 5.5/Apple/26. (DW: The End of the World)