Hmm this made me wonder why something like this wouldn’t melt the rock and then sink into the crust and then into the planet. Probably not hot enough.
And that made me think if we could build something like a big pellet of fissile material, encase it in tungsten or something so that it is hot enough to do so but remains stable, and then let it sink into the earth. Maybe that could be tracked? Then we could learn something about how it moves and where it ends up. But probably can’t be tracked since this isn’t star trek 🖖