Today in 1971, the United States conducted its largest-ever underground nuclear test. A Spartan antiballistic missile and its W71 warhead was lowered into a 7-foot-wide, 5,873-foot-deep shaft beneath Amchitka Island, Alaska. Yield was less than 5 Megatons.
The explosion raised the ground above the shaft 25 feet (it later collapsed into an almost mile-wide, 60-foot-deep subsidence crater). Then-AEC chairman James Schlesinger brought his wife and two daughters to watch. “It was kind of like a train ride” said nine‐year‐old Emily. 
The huge shockwave killed an estimated 900-1,100 sea otters, in some cases crushing their skulls. Other marine mammals died when their lungs ruptured or their eyes were blown out. Thousands of shore birds were killed, their legs pushed through their bodies and spines snapped.
A total of 30 Spartan and 70 short-range Sprint ABMs (armed with 1-kt W66 enhanced radiation warheads) were deployed at the Stanley R. Mickelsen Safeguard Complex at Nekoma, North Dakota, (which was only fully operational from Oct. 1975 to Jan. 1976) to protect Minuteman ICBMs.
