November 6
U.S. Conducts Largest Underground Nuclear Test Cannikin
The United States detonated its largest underground nuclear explosion to date on a remote Aleutian island, advancing anti-ballistic missile technology amid Cold War tensions and environmental controversy.
Summary
Amid Cold War tensions and the development of anti-ballistic missile systems, the United States Atomic Energy Commission prepared a major test on remote Amchitka Island in the Aleutians. The Cannikin shot, part of Operation Grommet, aimed to validate the W71 warhead for the Spartan missile. On November 6, the device with a yield of nearly 5 megatons was detonated at a depth of about 5,800 feet. The explosion registered 7.0 on the Richter scale, caused visible ground movement, and formed a large crater. Environmental concerns had sparked protests, including early actions by what became Greenpeace.
Context
By the late 1960s, the United States was developing anti-ballistic missile defenses in response to growing Soviet intercontinental ballistic missile capabilities. The Safeguard program, which included the LIM-49 Spartan interceptor, required a high-yield warhead capable of engaging incoming missiles at high altitudes. Nevada test sites proved inadequate for the scale of the required experiment, leading planners to consider remote locations with suitable geology.
What Happened
Amchitka Island in the Aleutians had hosted earlier tests, including the 1965 Long Shot experiment for seismic detection research. Preparation for the larger Cannikin shot, part of Operation Grommet, involved five years of work by staff from the Lawrence Livermore National Laboratory and the Atomic Energy Commission. A 1969 calibration test called Milrow confirmed the site's ability to contain a massive detonation. On November 6, 1971, the W71 warhead device, with a yield of nearly 5 megatons, was fired at the bottom of a shaft roughly 5,875 feet deep. The blast produced a seismic signal equivalent to magnitude 7.0, lifted the ground surface about 20 feet, and generated a subsidence crater that filled to form a lake more than a mile across.
Aftermath
The test triggered a series of smaller seismic events in the following weeks but caused neither the major earthquake nor tsunami that opponents had feared. Atomic Energy Commission officials, including Chairman James R. Schlesinger who witnessed the event with his family, declared the test a success and confirmed the warhead design. Environmental groups continued to criticize the operation, and the visible surface disruption reinforced concerns about long-term ecological effects on the island.
Legacy
Cannikin marked the high point of American underground nuclear testing in terms of yield and underscored the technical challenges of containing such powerful explosions. The protests surrounding the event helped launch Greenpeace as a major international environmental organization and contributed to growing public and political pressure that shaped later arms-control measures, including the 1974 Threshold Test Ban Treaty limiting underground test yields. The episode also illustrated the intersection of nuclear weapons development and emerging environmental regulation during the Cold War era.
Why It Matters
Cannikin represented the peak of U.S. underground nuclear testing scale and advanced ABM technology during arms control negotiations. It prompted stronger environmental activism and regulations on nuclear testing. The test underscored the environmental risks of such programs, influencing later treaties like the 1974 Threshold Test Ban Treaty limiting yields.
Related Questions
Why was Amchitka Island selected for the test?
Nevada sites could not safely accommodate an explosion of this size, and Amchitka offered remote location plus prior seismic testing experience.
What was the purpose of the W71 warhead?
It was designed for the Spartan anti-ballistic missile to intercept incoming warheads at high altitude as part of the Safeguard defense system.
How did the test contribute to the founding of Greenpeace?
Canadian activists protesting the shot chartered a vessel named Greenpeace; the organization formed during the campaign continued under that name.
Did the explosion cause a tsunami or major earthquake?
No large tsunami or catastrophic quake occurred, though the blast registered 7.0 and produced smaller aftershocks and visible ground movement.
What arms-control agreement followed Cannikin?
The 1974 Threshold Test Ban Treaty between the United States and Soviet Union limited underground nuclear tests to yields below 150 kilotons.
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America 250 Atlas: U.S. Conducts Largest Underground Nuclear Test Cannikin is part of U.S. presidential, constitutional, or national civic history.
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Sources
- Cannikin, Wikipedia. Accessed 2026-07-07.