Answer: Bacteria

In the 1950s and 1960s, the U.S. government was extremely worried about the impact of biological attacks on urban centers in the United States. In order to assess how widespread the damage of attacks on such sensitive and population-dense areas would be, they went to great lengths to conduct classified studies on large population centers.

In 1966, the U.S. Army released enormous amounts of aerosolized (but harmless) Bacillus globigii (also known as Bacillus subtilis) into the New York City subway system. Bacillus globigii is a gram-positive, catalase-positive bacteria commonly found in soil samples and, outside of cases where a patient has a severely compromised immune system, is completely harmless. After each release in the test, agents from the Army would study the subway platforms at set intervals to determine how long the bacteria survived, how long the area remained contaminated, and how many people would have been affected if the released substance was deadly.

The subway experiments were repeated in Chicago and were only one of over 200 different studies conducted across the U.S. during that time period.