Previously, a yield strength of 5,000 pounds per square inch (psi) was enough for concrete to be rated as “high strength,” with the best going up to 10,000 psi. The new UHPC can withstand 40,000 psi or more.
The greater strength is achieved by turning concrete into a composite material with the addition of steel or other fibers. These fibers hold the concrete together and prevent cracks from spreading throughout it, negating the brittleness. “Instead of getting a few large cracks in a concrete panel, you get lots of smaller cracks,” says Barnett. “The fibers give it more fracture energy.”
What material would that be? Corrosives have limits, they can’t just keep dissolving stuff forever.
And what would “total failure” look like? It’s a mountain, it’s not going to just collapse into goo.
Blood of xenomorphs from the Alien movies
That’s a work of fiction. You might as well suggest dropping lightsabres on the bunker.
Thus, the explosive assist for initial penetration. The type would depend on the composition of the concrete, you’d probably be more successful targeting the tension strength of the fibers or metals instead of the compression strength of the cement.
You don’t need goo, you just need enough weakening that it no longer supports the 250’ of loose rubble atop it and collapses into the interior space.
Coreium.
We drop an overloading nuclear reactor on top of the complex, and let the radioactive core China-Syndrome itself straight through, rendering the entire area uninhabitable for thousands of years.