When the colony ship almost traversed into a black hole, each of the ship's floors was affected by time dilation. Located at the top end of the ship, time travelled slower on floor 0000 than on any of the floors below. For example, 2 days and 10 hours at the top equated to just over 1000 years for those on floor 1056.
The ship was travelling from Mondas to pick up colonists. When the ship almost traversed into a black hole, around twenty crew members were sent to the bottom of the ship, approximately to the location of floor 1056, to reverse the rear thrusters. Unable to return to the top of the ship, the crew members remained at the bottom. (TV: World Enough and Time)
The Twelfth Doctor, deciding to see how far Missy's reformation had progressed, answered a spaceship distress call. He sent Missy out of the TARDIS to deal with it, having Bill Potts and Nardole acting as her companions. Jorj, a janitor left alone at the top of the ship, appeared. He panicked at the presence of a human, shooting Bill through the chest.
Bandaged figures came out of the lifts and took Bill's body downstairs. They told the others she would be "repaired", but not returned. After a short time (after knocking Jorj out and deducing the time differential) the Doctor, Missy and Nardole headed down after her. However, due to the time dilation, years passed for Bill. By the time they arrived on floor 1056, she had been converted into the first fully cybernetic humanoid. (TV: World Enough and Time)
Nardole stole a shuttlecraft and they all went up until crashing on floor 0507. They fought the pursuing Cybermen and evacuated the residents on that floor to a higher level, floor 0502. The Doctor and Bill stayed behind to hold off the Cybermen and destroy them all, though the Doctor was fatally injured from the battle. (TV: The Doctor Falls)
However, Heather found Bill through her tears and made her into a being of sentient oil like herself. They brought the Doctor back to his TARDIS on floor 0000 and Heather, able to pilot anything since her transformation, took off from the colony ship. (TV: The Doctor Falls)