[Narvin hesitates, but decides in the end that it's not exactly a secret.]

The Matrix. It's a supercomputer on Gallifrey that stores information on millions of years of observations around the universe. In addition to observing the present and remembering the past, it can also predict the future with relatively high accuracy. And it stores all the memories and brain patterns of every Time Lord who has ever died. It is, in essence, the closest thing we have to an afterlife, in addition to being a repository of all Gallifreyan knowledge.

It was destroyed during the recent civil war.