Aerial reconnaissance
The Liaison Office interviewed European refugee scientists and studied German physics journals. It compiled lists of German scientists of interest and possible locations of nuclear research and industrial facilities, and the mining and stockpiling of uranium and thorium ores. Little thorium was available in Germany or German-occupied Europe, and attention soon centered on the mines at Joachimsthal in Sudetenland (the German-annexed part of Czechoslovakia). Aerial reconnaissance was carried out periodically, and production was measured by assessing the size of the piles of tailings.