Bottle universe

A bottle universe was a phenomenon in which the entire contents of a universe were scaled down and placed within a bottle. The owner of a bottle universe had some degree of control over its contents, but many aspects took place of their own accord. (PROSE: Interference)

There was a series of bottle universes within bottle universes; many of which had versions of Time Lords inside them. (PROSE: Dead Romance, Interference - Book Two, Alien Bodies) Several times, Time Lords made attempts to migrate from their bottle universe to the bottle universe inside their bottle universe in order to escape from powerful danger. (PROSE: The Ancestor Cell, Dead Romance, Interference - Book Two)

The universe of the War
While meditating in Qixotl's ziggurat, Kortez journeyed into the astral plane and had a vision of the Eighth Doctor saying that their universe, the universe home to the War, was inside a bottle. (PROSE: Alien Bodies)

I.M. Foreman's bottle
I.M. Foreman created the bottle universe and played with the mechanics of it while on Foreman's World. It was stolen from her by the Time Lords while she and the Eighth Doctor slept. (PROSE: Interference)

Seventh Doctor's bottle
In Foreman's bottle, (PROSE: Dead Romance, Interference) the Seventh Doctor acquired a bottle universe by the time of his encounter with the Carnival Queen.

Chris Cwej and Marielle Duquesne discovered the bottle universe mounted in the corner of a brass room in the Doctor's TARDIS. They looked inside and saw the version of San Francisco inside the bottle; Cwej observed that the version of the Doctor inside the bottle had parked his TARDIS in San Francisco. (PROSE: Christmas on a Rational Planet)

Christine Summerfield's home universe
Another bottle universe was created by the sphinxes for the Gods of Dellah and secured by Chris Cwej while he was working for the Homeworld. Cwej opened a gateway for the Time Lords to invade and destroy the bottle universe version of 1970s Earth. (PROSE: Dead Romance) The Time Lords planned to use I.M. Foreman's bottle universe as a haven from the Enemy. Greyjan the Sane proposed the hypothesis that the Enemy were actually ancestor cells energised by the leaking bottle. (PROSE: The Ancestor Cell)

When Spring-heeled Jack looked into the Eighth Doctor's mind, he saw "a dull grey world in a bottle he couldn't wait to break". (COMIC: The Curious Tale of Spring-Heeled Jack)

Marnal's bottle
Marnal created a bottle universe when trying to discover the fate of Gallifrey after it had been destroyed. He later used it to track the Doctor through time and to also learn about what kind of person he was and was to become. (PROSE: The Gallifrey Chronicles)

Merging of bottle universes
After the Time Lords of the War universe stole Foreman's bottle universe, they kept it for safekeeping in the Time Vortex with plans to possibly colonise it to escape the Enemy. However, the four-dimensional nature of the Vortex caused a leak in the bottle and its universe began seeping out.

Kristeva heard loa whispering that the complete mixing of realities was inevitable. (PROSE: The Ancestor Cell)

Behind the scenes

 * In PROSE Interference and Dead Romance, author Lawrence Miles intended to use the bottle universes to establish the Virgin New Adventures (featuring the 7th Doctor and Bernice Summerfield) as a different universe from the BBC Eighth Doctor Adventures. Miles' connected the two novels (and three universes) with Father Kreiner: at the end of Interference - Book Two, Kreiner is put inside the Time Vortex of I.M. Foreman's bottle universe; Kreiner then appears within the Vortex of Chris Cwej's universe in Dead Romance and - after merging with the Horror - journeys into Christine Summerfield's universe.
 * However, in the Mad Norwegian Press release of Dead Romance, Miles admitted in the foreword that this was a bad idea and rightfully ignored.