Largest underground hydrogen bomb test - Project Grommet Cannikin

Cannikin was detonated on November 6, 1971, as the thirteenth test of the Operation Grommet (1971–1972) underground nuclear test series. The announced yield was "less than five" megatons – the largest underground nuclear test in US history. (Estimates for the precise yield range from 4.4 to 5.2 megatons). The ground lifted 20 feet (6 m), caused by an explosive force equivalent almost 400 times the power of the Hiroshima bomb. Subsidence and faulting at the site created a new lake, over a mile wide. The explosion caused a seismic shock of 7.0 on the Richter scale, causing rockfalls and turf slides of a total of 35,000 square feet. Though earthquakes and tsunamis predicted by environmentalists did not occur, a number of small tectonic events did occur in the following weeks, (some registering as high as 4.0 on the richter scale) thought to be due to the interaction of the explosion with local tectonic stresses.

According to wildlife surveys following the Cannikin event by the University of Alaska Fairbanks, 700–2,000 sea otters were killed by overpressures in the Bering Sea as a direct result of the explosion. This survey showed that number of sea otters endangered by the blast was far greater than the Atomic Energy Commission had predicted.