Did you know that our environment — birds, trees, even the rain — can betray us to talented spies? Neither did I, until I went to the Spy Museum.
In 1949, the Navy secretly collected rainwater at a station in Washington, D.C. (and also in Kodiak, Alaska) that they believed to have fallen from clouds that originated in the Soviet Union. As part of “Project Rain Barrel,” scientists at the Naval Research Laboratory analyzed the droplets to see if they had debris from nuclear weapons tests.
“A fallout of fission products from the air was detected in Kodiak, Alaska, Sept. 9,  by a gamma ray ground counter and in Washington, D.C. by filter paper on the same day,” they concluded.
The National Security Archive says the project “was critically important to forming the scientific consensus about the nature of the Joe-1 test,” which was the USSR’s first nuclear bomb test.