Yeah that's correct
Point is you never target a HDBT at its strong points . You target it's weaknesses like blast doors , entryways , ingress / egress tunnels for personnel , maintenance and supply , air ducts , ventilation points , escalators vents , support systems like cooling system , ventilation system , power supply points etc etc.
Thats why intelligence driven ops are carried out to identify vulnerabilities in HDBTs .
You primarily use aerial assets or space based assets for that including HUMINT .
Topography , terrain analysis , geological analysis , stratigraphy analysis are the tools that help to find points of vulnerability where correct application of force can lead to over pressure in adjoining areas leading to cave in and collapse of underground caverns.
Military victory is not all about target kill , it involves mission kill too.
If I can render a HDBT like a WMD storage location inoperable by destroying it's exits points , then no WMD will be deployable from that location and thus render it useless.
It is interesting that India is researching diamond-SiC . Their only utility is very high deformation resistance coupled with very high hardness.
Only single crystal diamonds are known to outperform them in the same .
Even at high hypersonic impact , the diamond-SiC will not deform beyond acceptable level necessary for penetration through very thick basalt rock layers.
Anyways American Thor orbital bombardment system with KE penetrators is a good idea but thing is it will wipe out everything like cities etc in proximity to the underground target through high seismic upheavals.