[dougk16]

Interesting, thanks for the link. Intuitively it still seems like much of the energy from the projectile would be absorbed deep in the Earth as it traveled through. For sure it would cause some damage, but it still seems like a nuclear bomb would have a higher effective energy, as far as the tactical goal of taking out a city or something.

[TheLoneWolfling]

You can adjust effective penetration depth by the "density" (read, mass per unit cross-sectional area) of the projectile.

You could go anywhere from an osmium needle through to a Mylar disk.

Another relevant XKCD: https://what-if.xkcd.com/20/