I found this by accident on google earth. It's pretty interesting and weird. I guess that people's best speculation is that it is a remnant from nuclear testing around the 60s.

Google earth URL https://earth.google.com/web/@40.01244676,-119.000836,1390.1...

This seems to be some other discussion about it: https://aair.smugmug.com/Aviation-Archaeology-1/Giant-concen...