Trenzalore

Trenzalore is a place where, according to Dorium Maldovar, the Doctor would visit at some point in his travels, which would coincide with 'the Fall of the Eleventh'. Maldovar also told the Doctor that at this time and place "no living creature can speak falsely or fail to answer." Here, the First Question in the Universe would be asked.

The Silence were determined that the Doctor never reach Trenzalore, so that the question would never be answered. It was a creed of the Silence that "Silence will fall when the Question is asked," or, as Maldovar translated it, "Silence must fall." (TV: Let's Kill Hitler, The Wedding of River Song)

The Eleventh Doctor and Clara Oswald visited Trenzalore, and each found out the truth about each other: who they were and their greatest secrets. (WC: She Said, He Said: A Prequel)

We can assume from an advert for the final episode in series 7, The Name of the Doctor, that Trenzalore is a place that the doctor should never go to.