One aspect of the geography of central Asia which intensively occupied Hedin for decades was what he called the "wandering lake" Lop Nur. In May 1934, he began a river expedition to this lake. For two months he navigated the Kaidu River and the Kum-Darja to Lop Nur, which had been filled with water since 1921. After the lake dried out in 1971 as a consequence of irrigation activities, the above-mentioned transportation link enabled the People's Republic of China to construct a nuclear weapon test site at Lop Nur.