What is the fine-grained topic of the following text: In 1879 – 82, when Tibet was a forbidden land for foreigners, Das went to Tibet twice in the disguise of a Buddhist lama crossing passes higher than 20,000 ft without the aid of modern mountaineering equipment and surveyed unknown regions of Kangchenjunga massif and Tibet on behalf of British Government of India. Only return the topic.
Das' covert exploration and survey of Tibet and Kangchenjunga massif for the British Government of India in the late 19th century.