Crosstie/Faultless was an attempt to find a safe testing area for large thermonukes exploded underground, since the NTS was mainly alluvial deposits unsuitable for a real hard shake. Had Faultless been successful, then a follow-on 5 MT test of the Spartan warhead would have taken place in the proposed "Adagio" test, about 2.7 miles south of Faultless. However, Faultless denied its name, finding faults which today causes the bore casing to stick 8 feet out of the ground (it was flush when drilled).
Because of the geology problem, they moved their sights to Amchitka Island in the west Alaskan Aleutians. Again, a ranging shot Mandrel/Milrow at 1
MT was tried, and two years later Grommet/Cannikin (5 MT) was executed. It is the only nuclear test that I'm aware of that actually set off tectonic
aftershocks. It was the largest underground test ever fired.
Crosstie/Faultless: 38.63421,-116.21623, 19 Jun 68
Adagio (cancelled): 38.59365,-116.21943
Mandrel/Milrow: 51.41575,179.17633, 2 Oct 69
Grommet/Cannikin: 51.47,179.1091, 6 Nov 71