Tibet was a former nation of central Asia occupied by China.
The Fifth Doctor travelled to Tibet in 1917, where he battled the Great Old Ones who attempted to be unleashed on Earth via his companion Erimem. (AUDIO: The Roof of the World)
The Second Doctor had previously visited Tibet in 1935 with Jamie McCrimmon and Victoria Waterfield, encountering the Great Intelligence for the first time. He also met a group of monks at the Det-Sen Monastery whom he helped to fight the Yeti of the Himalayas. (TV: The Abominable Snowmen)
Germany sent expeditions to Tibet in search of the occult sources of Nazi beliefs. (PROSE: Timewyrm: Exodus)
Victoria returned there decades later. (PROSE: Downtime)
K'anpo Rimpoche lived in Tibet for a time, learning the customs, the Tibetan language, and religion of that people. The Third Doctor also proved to be familiar with Tibetan customs. (TV: Planet of the Spiders)
The Eleventh Doctor, Amy Pond and Rory Williams visited Tibet while trying to get to Rio de Janeiro. (PROSE: The Glamour Chase)