Bees together. Honeybees (Apis melifera) on wild fennel. Credit: Jack Wolf. CC BY-ND 2.0. In the 1950s, scientists discovered that uranium and plutonium could fuel previously unimaginable explosions. Soon after, the US government developed and tested hundreds of nuclear weapons—first in the American West and later in the Pacific Islands. These nuclear tests caused blasts that released radioactive isotopes such as cesium 137 into the atmosphere. Later, in 1963, the United States signed the Treaty Banning Nuclear Weapons Tests in the Atmosphere, in Outer Space, and Under Water, after which it stopped testing nuclear weapons. But the environmental story does not end here as radioactive isotopes live on.