Here's what I have seen explained.

different kinds of underground bases. Some very old, natural cave systems. Some man made. Tunnel boring machines, nuclear powered. Low residue, advanced tech. Some big areas actually created BY nuclear bombs.

There are lots of things underground in the bases and tunnels link them. Some government stuff, some "elites" stuff, some "alien" stuff, some "demonic" stuff (may be the same thing?)

So there are a lot of bad dudes down there, and bad stuff being done. So, they're so big and such bad stuff that often the only way to "clean" the bases out is to nuke them.

So, when the good guys nuke an underground base it looks like an earthquake but doesn't have the usual signatures that indicate a natural earthquake. Also the depths are different.

There are some videos out there of massive tunnel entrances and one has a guy interviewing a trucker and the trucker is on one coast and says he'll go cross country completely underground.

Shrug. It's what I've heard. No sauce other than it's been talked about for years and years and years and makes sense, definitely plausible.