"The largest underground nuclear test conducted by the United States, Project Cannikin was one of three underground nuclear tests performed at different places on this 43-mile long island in the Aleutian Chain. This $200 million 1971 test was performed to test an Anti-Ballistic Missile warhead, for a Spartan ABM missile. It consisted of a 5 megaton-yield thermonuclear bomb, detonated in a 50-foot diameter chamber, at the bottom of a 5,875-foot shaft. The island of Amchitka was a military outpost in WWII, and the air field and base camp from that facility were reused for the nuclear testing program."
"0800041 - Project Cannikin Review - 1971 - 13:00 - Color - This video reviews Project CANNIKIN, a nuclear test conducted on Amchitka Island, Alaska, at 11:00 a.m., Bering Standard Time, on November 6, 1971. CANNIKIN, a slightly less-than-five-megaton device, was the largest underground nuclear test conducted in the United States. CANNIKIN was conducted to proof test a warhead for the Spartan missile, a Safeguard Ballistic Missile Defense Program.
The video shows the nuclear device and instrumentation canister being lowered into the shaft, detonation sequences, and test effects. A long-range view of water turbulence after the detonation is shown, but no tsunami or large ocean wave was observed or recorded. Numerous ground shock waves are shown at normal speed and as seen by high-speed, slow-motion cameras located at various sites on the island. Surface effects at ground zero and other island locations were filmed one day after the test. Approximately 38 hours after the test, a subsidence crater, approximately 1.5 miles in diameter and 55 feet deep, began to form.
Many scenes in the video have no sound intentionally; no material was deleted.
The three underground nuclear tests conducted on Amchitka Island, Alaska, were as follows:
LONG SHOT, October 29, 1965, shaft, Vela Uniform Project, approximately 80 kilotons
MILROW October 2, 1969, shaft, weapons related, approximately 1 megaton (Mt)
CANNIKIN, November 6, 1971, shaft, weapons related, less than 5 Mt"
Some cool facts:
First major project under the National Environmental Policy Act of 1969, which required the preparation of an “Official Environmental Impact Statement.”
Largest mined shaft in the United States with a single elevator to 6,000 feet.
Deepest 90-inch hole—6,150 feet.
Longest diagnostic canister—264 feet.
Largest off-continent diagnostic system—250 scopes with 100 percent data retrieval.
Largest load lowered downhole—over 400 tons.
Largest emplacement drill rig—1,000 ton mast.
First operational field computer system.
First successful downhole alignment with a laser beam.
First use of over 100 miles of downhole cables.
Largest cavity (52 foot diameter) mined through a mile-long shaft.
More info here:
It sounds like this was an enhanced radiation bomb, not unlike the tiny little one used for Sprint and HiBex. However, it may very well be that all modern nuclear devices are designed to not only dial a yield (that is pick the total explosive power) but also produce a chosen range of radiation amounts. Just as there are no fusion or fission bombs anymore, there may not be wholly enhanced radiation bombs anymore, just bombs along some point in a spectrum of nuclear effects. I do wonder if it would be possible to have removable layers of tamper that could not only alter yield, but also change the neutron flux or X - ray effects (particularly for space use in the latter.)
The flux coming out of a 5 megaton bomb, even a mile away, should do a great deal of damage to even a hardened warhead.