After stepping so close to the brink, both the U.S. and the USSR worked to reduce their nuclear tensions in the years immediately following. The most immediate culmination of this work was the signing of the Partial Test Ban Treaty in 1963, in which the U.S. and USSR agreed to no longer test nuclear weapons in the atmosphere, underwater, or in outer space. Testing underground continued, allowing for further weapons development, but the worldwide fallout risks were purposefully reduced, and the era of using massive nuclear tests as a form of saber rattling ended.