Plain of Pamir

The Plain of Pamir, which Marco Polo also called the Roof of the World, was an area of extremely high altitude in Central Asia. (TV: "The Roof of the World") The First Doctor, Ian, Barbara and Susan landed on the snowy plain immediately after fixing the TARDIS' faulty fast return switch. (TV: "The Brink of Disaster") There, in 1289, they met both Polo and Ping-Cho. The TARDIS broke down upon the plain, and was claimed for the glory of Kublai Khan. This meant that the foursome were forced to travel with the captured TARDIS, overland, from the plain to Peking. (TV: "The Roof of the World")

Behind the scenes
In the real world, there is no such thing as the Plain of Pamir. This was evidently just a name that Polo used for the precise place where the TARDIS landed. However, it is used consistently, both in the televised and novelised versions of the story — instead of the real world terms, "Pamir", "Pamirs", or "Pamir Mountains".