You might have thought that there was some giant underground complex, but it's really just mining equipment drilling a hole and a bunch of sensors strung out feeding data to recording devices that are a long way away.
https://en.wikipedia.org/wiki/Underground_nuclear_weapons_te...
Edit: to clarify, they don't reuse holes for repeat testing (that I'm aware of), they just drill a new one a few miles apart. So a "testing site" is just a bunch of land that the government owns and is willing to never use it for anything else.
Still! What does the container for a nuclear explosion look like on the inside? Is it smooth and glassy? Does molten material drip down and form stalactites and stalagmites? Does the surface crack as it cools? What color is everything?
Here's Project Gnome's: image https://en.wikipedia.org/wiki/Operation_Nougat#/media/File:P... better color https://www.reddit.com/r/HistoryPorn/comments/1fgvt8/a_perma... video https://www.youtube.com/watch?v=DFJ2MyWlXgs&t=1442 report with diagrams https://www.osti.gov/biblio/4612556 cross-section http://nuclearweaponarchive.org/Usa/Tests/GnomeDiag1024.gif