Around the world, an international monitoring system (IMS) detects signs that a nuclear weapon could have been detonated. Later, in 1955, United States Operation Wigwam conducted a single underwater nuclear test at a depth of 600 m to determine the vulnerability of submarines to nuclear explosions. One of the radionuclide technologies employed by IMS detects ultratace levels of radioactive noble gas xenon, a by-product of nuclear explosions. Underground nuclear testing was banned by the 1996 Comprehensive Nuclear-Test-Ban Treaty (CTBT), which bans all nuclear explosions anywhere and by anyone.
The first underwater nuclear test, 'Baker' (video), was conducted by the United States in 1946 at its Pacific Proving Grounds in the Marshall Islands for the purpose of evaluating the effects of nuclear weapons used against naval ships. There are many different ways to detect a nuclear detonation, including seismic, hydroacoustic and infrasound detection, air sampling, and satellites. PNNL participates in a large national research enterprise, known as the low-performance nuclear monitoring program, which is designed to improve the ability to monitor potential evidence of evasive nuclear explosives through scientific research. It is important to monitor the world for these explosions because some countries may conduct nuclear explosive tests to develop or improve nuclear weapons.
Despite these measures, there is a limit to the sensitivity of underground and submarine systems, so very small nuclear explosions at a great distance from receiving sites may go undetected or misidentified as a small earthquake. Some studies and evaluations, including an evaluation by Arjun Makhijani of the health effects of nuclear weapons complexes, estimate that cancer deaths due to global radiation doses from nuclear test programmes in the atmosphere of the five nuclear-weapon States amount to hundreds of thousands.