After the ban on surface tests of nuclear devices due to the increased radiation (read article on Nuclear Test Ban Treaties) there was still one way to test the functionality of an atomic bomb: underground testing.
Underground testing was allowed, provided that it does not cause “radioactive debris to be present outside the territorial limits of the State under whose jurisdiction or control such explosion is conducted.” (wikipedia.org, 5.11.14)
The idea behind it lies in burying the nuclear weapon device at sufficient depth so the explosion may be contained and no radioactive materials will be released to the atmosphere. Click here for a nice graphic by Thomson Reuters.
The effects of an underground nuclear test is depending on the depth and yield of the explosion, as well as the nature of the surrounding rock. When the bomb is detonated enough deep there is no venting of gases or other contaminants to the environment. The test ist then said to be contained.
The first underground tests by the U.S. were conducted in the fifties and were considered as cratering or bunker-buster weapons while aboveground testing still was conducted – the bombs were released in an open shaft that allowed venting – called “roman candle” style. With the “Rainier” test in 1957 there were two successful objectives: No radioactivity was released in the atmosphere and diagnostic information can be obtained from underground tests.
With the Limited Test Ban Treaty in 1963 , the U.S. government started to improve containment techniques – with some accidental releases of radioactive material, most of them only measurable within the Nevada Test Site. But a test codenamed “Baneberry” in 1970 led to a massive venting of radioactive material which was tracked as far as the Canadian border – and resulted in new testing procedures with focus on minimizing even the most remote chance of an accidental release of radiation.
On the Nevada Test Site in a 65 miles distance to Las Vegas (the mushroom clouds could be seen from the downtown hotels and became tourist attractions), there have been 928 announced nuclear test, 828 of them were underground. The aerial view of the test side shows numerous subsidence craters and resembles a swiss cheese.