From Wikipedia

Tibets Hidden Kingdom

Tibet has attracted European missionaries and explorers for over 500 years. The location of Tibet deep in the Himalaya mountains made travel to Tibet extraordinarily difficult at any time. In addition, the internal and external politics of Tibet, China, Bhutan, Assam, and the northern Indian kingdoms continually conspired to make entry into Tibet politically difficult for all Europeans. The combination of inaccessibility and political sensitivity made Tibet a mystery and a challenge for Europeans well into the twentieth century. These obstacles did not deter a number of missionaries, scholars, geographers, soldiers and mystics from making the attempt.