During his expedition of 1907–1909, Kozlov explored the Gobi Desert and discovered the ruins of Khara-Khoto, a Tangut city destroyed by the Ming Chinese in 1372. It took him several years to excavate the site and bring to St. Petersburg no fewer than 2,000 books in the Tangut language he uncovered there. Kozlov described his findings in a large volume entitled Mongolia and Amdo and the Dead City of Khara-Khoto (1923). He was awarded the 1911 Royal Geographical Society's Founder's Medal for his explorations.
