Solar Jetman is the only game I can think of that does essentially the same thing as Sub-Terrania and it's even harder.
I just got Galak-Z recently and it made me think a bit about Sub-Terrania in that it's a 360 degree shooter where you travel through caverns. It doesn't have the downward gravity but the ship retains momentum like in Asteroids.
Thanks for the suggestions. Giantbomb lists Pixeljunk Shooter as a similar game, and even though I have both games I still haven't played either. *checks Youtube footage*
My random brain remembered a random game for this old topic.
Cybernoid is pretty similar to Subterrania. Actually I'm willing to bet Subterrania took a lot of inspiration from it. Cybernoid was released on a ton of platforms, including ZX Spectrum, Atari ST, Amstrad CPC, C64, NES, Amiga, and Virtual Console.