Well, alright then. I guess they’re just making neutron star bombs down there.Precisely. Gravity manipulation could do that. Very hypothetical, of course. Then again, thargs seem to have reactionless drives and graviitics seem to be the best bet for solving that little problem. Frame shift drives already bend spacetime, albeit in a quite crude way.
Dissipation is the problem. The BH eating everything around them, not so much. Feeding a micro black hole is more difficult than it seems; their event horizons have diameters way smaller than atoms ( eg, around 3e-6 pm for a BH that masses 1 000 000 ton and 3e-8 pm for a 10 000 ton BH; an hydrogen atom has a diameter of around 74 pm). Quite hard to keep a micro BH well-fed so it doesn't evaporate very energetically due to Hawking radiation.
Jokes aside, and as interesting of a thought as it may be, I favor the idea that there’s just a gigantic sub-surface structure underneath the spires(and that they haven’t all yet surfaced fully), to explain all of that mass.
What will it be? Who knows. Probably not good, though.