Possibility Tree

The Possibility Tree, (AUDIO: Auld Mortality) also known as the Great Tree (PROSE: The Hunting of the Slook) or Tree of Infinity, (PROSE: Cat's Cradle: Time's Crucible) was an aspect of Time as an infinitely large tree composed of all possibilities branching from every moment and forming the omniverse. (PROSE: The Hunting of the Slook, AUDIO: Auld Mortality)

Description
Since the beginning of time, (PROSE: The Hunting of the Slook, AUDIO: The Thief Who Stole Time) infinite possibilities were created from every moment, branching tree-like in "all directions" (PROSE: Battlefield, AUDIO: Auld Mortality, The Thief Who Stole Time) to become the omniverse. (PROSE: The Hunting of the Slook, AUDIO: The Thief Who Stole Time) Having an affinity for plantlife, Time thus existed in this aspect of herself as a huge gnarled tree with flowering branches. While containing all reality within itself, the Tree was under a clear sky blotted out by its branches and on the surface of a shaded ground thick with the roots of Time. (PROSE: The Hunting of the Slook)

Time's music resonated forever through the universes (PROSE: The Hunting of the Slook, AUDIO: The Gift) which made up the tree's branches. (PROSE: The Hunting of the Slook)

The Doctor's Possibility Generator was connected to the relative dimensions of Imagination and Possibility theory, with the latter bringing him in contact with the Possibility Tree. In a generated garden of Time's roses, the Doctor was able to look down on the entirety of the Possibility Tree, seeing unlimited worlds spiraling off from branches. Unlike in other parts of the Possibility Generator, visitors to the Tree would themselves begin to branch into infinite possibilities. (AUDIO: Auld Mortality)

Known branches
Amongst the locations the Doctor showed Susan in the tree were the fires of the Aurora Temporalis ("the anvils of Heaven from which all time springs"); the Frost Fairs of Ice-Askar the Winter Star, from which you "skate through the sky and carve sculptures in the clouds"; and the glittering torchlight on the canals of Venice. One could see in the tree "cities and jungles, alien kings and alien creatures. Always darkness, and light, in perpetual battle."

As well as this, the Doctor also could see alternate Gallifreys: one in which they were as steeped in blood as they were currently steeped in dust; one where the the world was dead and all the children died; and one where Gallifrey was ruled by sorcery, not science.

He also, sometimes, thought he could see himself, travelling from world to world in an old TARDIS, just like he always wanted too. (AUDIO: Auld Mortality)

History
The Doctor said you could become blinded by the sheer scale of the diverse worlds. He was content to stay in the Possibility Tree forever and never go outside, but Susan convinced him otherwise and carefully brought him down through the branches and twigs to reality. (AUDIO: Auld Mortality)