The northwest corner of the Nevada National Security Sites, sometimes referred to as the Nevada Test Site, overlaps the HMA.
The map at Wikipedia indicates it’s Area 20, with one red dot in the upper left corner.
The National Data Viewer shows a crater in that area.
The measurement tool puts the diameter at slightly less than one thousand feet.
Western Horse Watchers does not know when the detonation occurred, if the horses can access the area, and how much radiation is present.
Western Horse Watchers Association 