Forbidden territory: on horseback to the source of one of Earth’s most formidable rivers

In this extract from his new book, Colin Thubron is in Genghis Khan country, tracking the source of the mighty Amur in Mongolia

Across the heart of Asia, at the ancient convergence of steppe and forest, the grasslands of Mongolia move towards Siberia in a grey-green sea.

The land’s silence is almost unbroken. It is barely inhabited. At its farthest reach, near the Russian frontier, almost 5,000 square miles are forbidden to travellers. These mountains, once the homeland of Genghis Khan, are today a near-sacred wilderness. The solitary track that reaches them ends at a barrier and a rangers’ lodge. And here we wait – a guide, two horsemen and I – to enter a region that none of us truly knows.