re: Bunker buster technology
What needs to be looked for are cracks or "geysers" where material has possibly been brought up from deep below to freeze on the surface. Then the problem becomes (relatively) easier: penetrating deep enough into the frozen outflow field to get to the stuff that's deep enough so that radiation and other large-molecule-busting effects can't get to it. Don't know how deep that would be, but it would be a hell of a lot less than 12 miles.
Same as analyzing the Earth's interior by examining lava flows, not by drilling a hole through the MoHo.