These were said to be tests where uranium was detonated underground in quantities insufficient to create a nuclear explosion, but enough to provide useful data for nuclear weapon modeling. I vaguely recall that these were done from the 1970s to the 1990s at least. Does actions have any information about this?
The only underground testing done in Nevada, to my knowledge, was at the Nevada Test Site. This is a large area within the Nellis Air Force Gunnery and Bombing Range; it is a military site. Now whether someone wants to argue that this is, in some historical or ethical sense, "American Indian land" (one can certainly make that argument, I am sure, of much of the American southwest, if not most of the Americas), legally this has been federally owned since 1940, and managed by the government since the 1930s.
Subcritical testing is definitely something that has been done, and is still done, at the Nevada Test Site.
I'm not sure which angle of your question you are most interested in — the question of where it was done, or the question of what was done.