Tibet

From TARDIS Index File, the free Doctor Who reference.

Tibet is a former nation of central Asia, now occupied by China.

The Doctor travelled to Tibet in 1917, where he battled the Great Old Ones who attempted to be unleashed on Earth via his companion Erimem. (BFA: The Roof of the World)

The Doctor had previously visited Tibet in 1935 with Jamie McCrimmon and Victoria Waterfield, encountering the Great Intelligence for the first time. (DW: The Abominable Snowmen) Victoria returned there decades later (MA: Downtime).