Mount Lung

 was the name of a mountain on Gallifrey, according to the stories of Lady Nine Teeth. It was also known as Cadon in the Prydonian Academy, and Lung by the Doctor's cousins. The Capitol could be seen from its summit.

The First Doctor accompanied his mentor K'anpo Rimpoche to the mountain before he left Gallifrey for the first time.

He later met him there again, shortly before his regeneration into his second incarnation, where K'anpo convinced the Doctor that he was already a very different person from his younger self, and that regeneration would not be so drastic a change as he thought. (PROSE: The Three Paths)