Hm. Caverns.
DUMBs, I hear they are called.
Deep Underground Military Bases.
Filled with Reptilians.
Could tell that LANL in New Mexico back in the sixties worked on the idea of tunneling by the melting rock and forming it into an integral glass liner in one fell swoop. That likely is one of the origin sources for the rumors of networked underground military bases.
Some enterprising souls worked up the requirements for a fission power source to deliver the required heat at a temperature suitable for a vehicle called a Sub-Selene for the Lunar Colonization guys. They already had the ideas in play for using ceramics for nuclear rockets.
Why not throw in underground aliens?