I was also thinking about this and I think it has to go a step further: Assets don't just need to be underground, they need to be on mobile rails underground. They need to constantly be moveable and pop out of one of thousands of holes to attack or if it comes to defensive e.g SAM sites they need to be moveable so that when an incoming missile is not interceptable that it can simply move away to a different underground location, pop out somewhere else and be able to keep defending. All you should be losing when a missile hits is a one of the underground exit holes. And of course to defeat such underground networks you need vast armies of small intelligent drones that can go in there and explore every tunnel where no human wants to risk setting foot in.
The cost of building all the tunnels is astronomical. Making tunnel boring cheap, fast, and resilient to different soil types + rock would also pay huge dividends for urban mass transport (subway lines).
This was the concept of the mx missile program (peacekeeper icbm) - the shell game concept was to have miles and miles of tracks with multiple launch sites, and to consistently move live and dummy missiles between them - that way there was a large number of potential targets and uncertainty as to which targets contained live missiles - I think the Chinese are now building a similar concept to this in the Gobi desert (if memory serves).
> And of course to defeat such underground networks you need vast armies of small intelligent drones that can go in there and explore every tunnel where no human wants to risk setting foot in.
In the book they're defeated by biological warfare.
