Pocket universe (Protect and Survive)

Two Elder God agents of Moloch once created a pocket universe in which they preserved the original timeline of Earth, allowing them to create a new timeline in which the planet was destroyed by nuclear war

The Seventh Doctor managed to alter this pocket universe so that it instead contained 5 miles of the alternate timeline created by Elder Gods. He then tricked the pair of Gods into inhabiting two individuals in the universe, trapping them there, where they were forced to live out the same 9 days until they could prove that they'd learnt their lesson. After the nine days had passed, from 9-18 November 1989, the universe time looped.

Ace and Hex arrived there by mistake, where they had to live out the full 9 days with the Elder Gods. The Gods managed to use them to escape, with the help of Moloch. Ace and Hex also managed to escape by passing the test set by the Doctor for the Elder Gods. (AUDIO: Protect and Survive)