Underground Great Wall of China (PLA Rocket Force): formally state-acknowledged 2009. Georgetown University analysis (Philip Karber 2011) documents 5,000+ km tunnel network. Purpose: transport, store, conceal ICBMs (DF-5 class) enabling mobile survivability against first strike + robust second-strike capability. One country, fully documented.
Yamantau Mountain (Russia, Southern Urals): identified by US intelligence + satellite imagery 1990s. ~400 square miles footprint. 1996 NYT + Gen. Eugene Habiger: 'millions of square feet available for underground facilities.' 1997 US intelligence: extensive excavation + connecting rail lines. Designed to ensure survival of Russian political + military elite during nuclear exchange. Fully corroborated.
Engine read: the confirmed international examples establish that nation-state-scale subterranean architecture is technically + economically feasible. The US / Five-Eyes capability-baseline for analogous work is at minimum equivalent to China's + Russia's. The question shifts from 'is such construction possible' to 'did the US build its analogues covertly, at what scale, and where?'