Say an advanced alien civilization has mastered the manufacture and use of super-dense neutronium. They utilize it as the “armored shell”, for lack of a better word, of a military armored vehicle, and have a (magical?) engine that can propel the vehicle at combat speeds.
I read, years ago, Greg Bear’s THE FORGE OF GOD, where aliens dropped neutronium and anti-neutronium onto the earth and the two masses tunneled into earth and eventually met at the core where they went boom. I’m writing a story and came up with this brainstorm—the aliens cut the power to this neutronium car, and it sinks into the ground to a certain depth that they require and they feather up the power to maneuver as they wish. Like a submarine, except through rock, soil, etc. Voila! A different kind of stealthy vehicle! Plus, above gound (AIU)I, any bad guys who even touched the car would rue the day. Come to think of it, it would only have to touch or ram any other vehicle, eh? Or any barrier for that matter. Useful, no?
As I have little military experience, do you think, used in this stealthy, underground manner, that this would be tactically useful? Forex, cruising a couple hundred meters underground, able to pop up at any time or location? Especially against foes of inferior tech, like our present day?
My question is not how such a vehicle would be made. The question is: the tactic of sub surface assets that can maneuver underground as well as our submarines do in the ocean.
Or fogetaboutit. Try something else. :)