Weirdly -- for this conversation -- i worked briefly and boredly, in the USAF Acquisition and Modernization shop for ICBMs during the late 90s. This was a very painful time for me professionally. They basically hired me to write a policy paper to promote the ease in which a conventionally-armed Minuteman III could be fired off to destroy some tunnel in Iran or Iraq or whatever. I had to research Russian and US arms control agreements and the policy status of those agreements. A lot of focus on the trashed state of Russian ballistic missile warning satellites at the time. Basically, we could have launched anything we liked at the time and there was no other entity in the World that would be able to track it -- besides the US.
There were much better ways to attack underground targets. Something falling at those speeds from space were actually not going to be effective for all sorts of physics reasons.