It was a huge tree in the garden, which according to the Doctor one could see unlimited worlds spiralling off from branches. Amongst the locations the Doctor showed Susan) in the tree was 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.
The Doctor said you could become blinded by the sheer scale of the diverse worlds". He was content to stay in there forever, and never go outside, but was convinced otherwise by Susan, who carefully brought him down through the branches and twigs to reality. (AUDIO: Auld Mortality)