On September 19, 1957, a 1.7-kiloton nuclear device called Rainier was detonated at the end of a 1,600-foot tunnel, 900 feet below the surface of Rainier Mesa in the Nevada Test Site. It was the first American underground test that contained its radioactivity. The shot was part of Operation Plumbbob, which ran 29 explosions that summer, most of them in the open air.
The reason was fallout. By 1957 the United States and the Soviet Union had fired more than 200 atmospheric tests between them, and strontium-90 from them was measurable in milk and in children's teeth across the northern hemisphere. The Castle Bravo shot of March 1954 had produced fallout that killed one Japanese fisherman, sickened 23 more, and contaminated the Marshall Islands. Public and diplomatic pressure was building, and the test ban negotiations in Geneva were under way. Containment was the technical answer, and Rainier was meant to show it could be done.
The design was simple. The device sat in a chamber at the end of a horseshoe-shaped tunnel, which was then backfilled with sandbags and gravel so that the blast would stop within a few hundred feet. The rock above was welded tuff, and the cavity was expected to collapse and trap the fission products. It worked: the explosion was barely felt on the mesa, there was a small subsidence crater afterwards, and the radiation that escaped was a fraction of what an atmospheric shot of the same yield would have released.
What it established was a method rather than a weapon. Rainier was a test of containment, not of a new design, and it was followed by shots in vertical shafts, which were cheaper and easier to seal. Between 1957 and 1992 the United States fired 815 underground tests at the Nevada site, and the Soviet Union, Britain, France and China did the same at their own. The Partial Test Ban Treaty of 1963 banned atmospheric, underwater and space testing, and was only negotiable because containment had been shown to work.
The site is still radioactive. Rainier Mesa was the main area for tunnel shots, and the ground above the cavities is monitored. The United States stopped testing in September 1992 and has signed but not ratified the Comprehensive Test Ban Treaty of 1996. Stockpile maintenance is now done without full-scale explosions.