Deep time

Even before taking control of history and becoming Time Lords, the Great Houses of the Gallifreyans sent pioneers to explore "deep time", or "deep-time". (PROSE: Cold Fusion, The Book of the War)

Patience's husband, seemingly a version of the Doctor before the "First", was "a pioneer and leader among [the Doctor's] people, one of the first Gallifreyans to enter the Time Vortex after it was discovered". He notably "led an expedition into deep time", returning with "travellers' tales of monsters and lost civilisations". (PROSE: Cold Fusion) As recounted in The Book of the War, prior to the anchoring of the thread, "early deep-time explorations performed by the Great Houses' pioneers" showed that there were "things at work in the formative future, things which simply couldn't be classified or even monitored by the Houses' own technology". This contributed to the Great Houses' decision to become lords of history. (PROSE: The Book of the War)

The Rassilon Imprimatur was responsible for mapping Time Lords' biodata onto the vortex as pure mathematics, (PROSE: Interference) protecting them from deeptime phenomena. (PROSE: The Book of the War) Sabbath wrote that it tethered a person like the Doctor to "his own realm", and he called it "the Great Eye". Without this tether, (PROSE: The Adventuress of Henrietta Street) humans like Sabbath had difficulty traveling in Deep Time, although the Doctor's human companions were protected by the TARDIS. (PROSE: Camera Obscura)

Sabbath described the experience of traveling into the deeper realms of time unprotected as "a lack of cohesion, a certain lack of integrity… My sense was that my own world no longer acknowledged me and as a consequence I was ceasing to be." Similarly, Juliette Vierge worried regarding her travels with Sabbath that "one could barely see such things and still consider oneself to be a person, as such." (PROSE: The Adventuress of Henrietta Street) When Umbaste dropped his bio-imprimaturs and exposed himself to the deeptime phenomena of the caldera, he spent a year in a fugue state. (PROSE: The Book of the War)

The Seekers, which lived for billions of years, experienced the universe from the perspective of the geologic vastness of Deep Time. From this perspective, the birth and death of stars was like the life of a butterfly or the flickering of a candle. In Deep Time, gravity would outlast matter. (PROSE: Beltempest)

The Phaeron disposed of the Glamour in Deep Time by creating a wormhole that only existed in one spatial dimension. It went backwards in time, deeper and deeper, to before the Phaeron had existed. To ensure that the Glamour would be forever lost, the Phaeron accompanied it into Deep Time, even though they also would never be able to return. (PROSE: Deep Time)

Having "deep-time memory" meant a person could remember events from past iterations of history, even if they had been overwritten. This was the only way that some agents of the Great Houses could remember the academy on the Homeworld which the Celestis later transformed into Mictlan.

At the start of the War in Heaven, forces of both the Homeworld and the Enemy emerged from deep-time enclaves to initiate their attacks. During the War House Arpexia's specialists created more deep-time strategies and weapons systems than any of the other Houses. (PROSE: The Book of the War)

Behind the scenes

 * "Deep time" is an analogue of the phrase "deep space".