well, governments have been building underground bunkers since the invention of bombs.
Having said that, some of the things I have seen passed off as "secret underground bunkers" are laughable.
The video clip from Jesse Ventura's new mind-suck of a show is a good example. They drive out into the middle of nowhere, only to be confronted by a security guard, who says that they need clearance from "Northrup Security".
Northrup is a defense contractor. They build planes and missiles. If they left their stuff laying around on the surface where (for example) the Iranians or Russians or whoever could get a peek at it, they would be "defense contractor" no more.
Does that qualify as a 'secret underground bunker'? A place with a visible entrance and a sign by the driveway declaring that it is "Northrup-Grumman"?
How about NORAD? That's definitely and underground bunker, but 'secret'? Not so much.
Here's the thing… I expect the government to have "secret underground bunkers". That's part of what governments do. I expect that the UK has them, that Australia has them, that North and South Korea has them, and that Russia has them, and India, and Pakistan, and Germany, etc., etc.
There's an old saying in military circles: You plan for an enemy's capacity, not their intent. There's another old saying in military circles: Anyone not "us" is "them".
I seriously doubt that people promoting this idea that there has been a sudden surge in bunker-building can prove their case.
Do 'secret underground bunkers' exist? Yes they have, since about as long as there has been war. I suppose that a tribe of cave-men hiding their women and children in a cave, and then concealing the entrance counts too.